Messages in this thread Patch in this message | | | Date | Thu, 5 Nov 2020 15:20:07 -0500 | From | Konrad Rzeszutek Wilk <> | Subject | Re: [PATCH v3] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests. |
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On Thu, Nov 05, 2020 at 07:38:28PM +0000, Ashish Kalra wrote: > On Thu, Nov 05, 2020 at 02:06:49PM -0500, Konrad Rzeszutek Wilk wrote: > > . > > > > Right, so I am wondering if we can do this better. > > > > > > > > That is you are never going to get any 32-bit devices with SEV right? That > > > > is there is nothing that bounds you to always use the memory below 4GB? > > > > > > > > > > We do support 32-bit PCIe passthrough devices with SEV. > > > > Ewww.. Which devices would this be? > > That will be difficult to predict as customers could be doing > passthrough of all kinds of devices.
But SEV is not on some 1990 hardware. It has PCIe, there is no PCI slots in there.
Is it really possible to have a PCIe device that can't do more than 32-bit DMA?
> > > > > > > Therefore, we can't just depend on >4G memory for SWIOTLB bounce buffering > > > when there is I/O pressure, because we do need to support device > > > passthrough of 32-bit devices. > > > > Presumarily there is just a handful of them? > > > Again, it will be incorrect to assume this. > > > > > > > Considering this, we believe that this patch needs to adjust/extend > > > boot-allocation of SWIOTLB and we want to keep it simple to do this > > > within a range detemined by amount of allocated guest memory. > > > > I would prefer to not have to revert this in a year as customers > > complain about "I paid $$$ and I am wasting half a gig on something > > I am not using" and giving customers knobs to tweak this instead of > > doing the right thing from the start. > > Currently, we face a lot of situations where we have to tell our > internal teams/external customers to explicitly increase SWIOTLB buffer > via the swiotlb parameter on the kernel command line, especially to > get better I/O performance numbers with SEV.
Presumarily these are 64-bit?
And what devices do you speak off that are actually affected by this performance? Increasing the SWIOTLB just means we have more memory, which in mind means you can have _more_ devices in the guest that won't handle the fact that DMA mapping returns an error.
Not neccessarily that one device suddenly can go faster.
> > So by having this SWIOTLB size adjustment done implicitly (even using a > static logic) is a great win-win situation. In other words, having even > a simple and static default increase of SWIOTLB buffer size for SEV is > really useful for us. > > We can always think of adding all kinds of heuristics to this, but that > just adds too much complexity without any predictable performance gain. > > And to add, the patch extends the SWIOTLB size as an architecture > specific callback, currently it is a simple and static logic for SEV/x86 > specific, but there is always an option to tweak/extend it with > additional logic in the future.
Right, and that is what I would like to talk about as I think you are going to disappear (aka, busy with other stuff) after this patch goes in.
I need to understand this more than "performance" and "internal teams" requirements to come up with a better way going forward as surely other platforms will hit the same issue anyhow.
Lets break this down:
How does the performance improve for one single device if you increase the SWIOTLB? Is there a specific device/driver that you can talk about that improve with this patch?
> > Thanks, > Ashish' > > > > > That is the right thing being something less static. > > > > Can you work with me on what that could be please? > > > > > > > > Thanks, > > > Ashish > > > > > > > What I wonder is if we can combine the boot-allocation of the SWIOTLB > > > > with the post-boot-allocation of SWIOLTB to stitch together > > > > continous physical ranges. > > > > > > > > That way you have the flexibility at the start of using 64MB but if there > > > > is pressure, we grow to a bigger size? > > > > > > > > > > > > > > Thanks, > > > > > Ashish > > > > > > > > > > > > memory. > > > > > > > > > > > > > > Using late_initcall() interface to invoke > > > > > > > swiotlb_adjust() does not work as the size > > > > > > > adjustment needs to be done before mem_encrypt_init() > > > > > > > and reserve_crashkernel() which use the allocated > > > > > > > SWIOTLB buffer size, hence calling it explicitly > > > > > > > from setup_arch(). > > > > > > > > > > > > > > The SWIOTLB default size adjustment is added as an > > > > > > > architecture specific interface/callback to allow > > > > > > > architectures such as those supporting memory > > > > > > > encryption to adjust/expand SWIOTLB size for their > > > > > > > use. > > > > > > > > > > > > > > Signed-off-by: Ashish Kalra <ashish.kalra@amd.com> > > > > > > > --- > > > > > > > arch/x86/kernel/setup.c | 2 ++ > > > > > > > arch/x86/mm/mem_encrypt.c | 42 +++++++++++++++++++++++++++++++++++++++ > > > > > > > include/linux/swiotlb.h | 1 + > > > > > > > kernel/dma/swiotlb.c | 27 +++++++++++++++++++++++++ > > > > > > > 4 files changed, 72 insertions(+) > > > > > > > > > > > > > > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c > > > > > > > index 3511736fbc74..b073d58dd4a3 100644 > > > > > > > --- a/arch/x86/kernel/setup.c > > > > > > > +++ b/arch/x86/kernel/setup.c > > > > > > > @@ -1166,6 +1166,8 @@ void __init setup_arch(char **cmdline_p) > > > > > > > if (boot_cpu_has(X86_FEATURE_GBPAGES)) > > > > > > > hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT); > > > > > > > > > > > > > > + swiotlb_adjust(); > > > > > > > + > > > > > > > /* > > > > > > > * Reserve memory for crash kernel after SRAT is parsed so that it > > > > > > > * won't consume hotpluggable memory. > > > > > > > diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c > > > > > > > index 3f248f0d0e07..e0deb157cddd 100644 > > > > > > > --- a/arch/x86/mm/mem_encrypt.c > > > > > > > +++ b/arch/x86/mm/mem_encrypt.c > > > > > > > @@ -489,7 +489,49 @@ static void print_mem_encrypt_feature_info(void) > > > > > > > pr_cont("\n"); > > > > > > > } > > > > > > > > > > > > > > +#define TOTAL_MEM_1G 0x40000000UL > > > > > > > +#define TOTAL_MEM_4G 0x100000000UL > > > > > > > + > > > > > > > +#define SIZE_128M (128UL<<20) > > > > > > > +#define SIZE_256M (256UL<<20) > > > > > > > +#define SIZE_512M (512UL<<20) > > > > > > > + > > > > > > > /* Architecture __weak replacement functions */ > > > > > > > +unsigned long __init arch_swiotlb_adjust(unsigned long iotlb_default_size) > > > > > > > +{ > > > > > > > + unsigned long size = 0; > > > > > > > + > > > > > > > + /* > > > > > > > + * For SEV, all DMA has to occur via shared/unencrypted pages. > > > > > > > + * SEV uses SWOTLB to make this happen without changing device > > > > > > > + * drivers. However, depending on the workload being run, the > > > > > > > + * default 64MB of SWIOTLB may not be enough & SWIOTLB may > > > > > > > + * run out of buffers for DMA, resulting in I/O errors and/or > > > > > > > + * performance degradation especially with high I/O workloads. > > > > > > > + * Increase the default size of SWIOTLB for SEV guests using > > > > > > > + * a minimum value of 128MB and a maximum value of 512MB, > > > > > > > + * depending on amount of provisioned guest memory. > > > > > > > + */ > > > > > > > + if (sev_active()) { > > > > > > > + phys_addr_t total_mem = memblock_phys_mem_size(); > > > > > > > + > > > > > > > + if (total_mem <= TOTAL_MEM_1G) > > > > > > > + size = clamp(iotlb_default_size * 2, SIZE_128M, > > > > > > > + SIZE_128M); > > > > > > > + else if (total_mem <= TOTAL_MEM_4G) > > > > > > > + size = clamp(iotlb_default_size * 4, SIZE_256M, > > > > > > > + SIZE_256M); > > > > > > > + else > > > > > > > + size = clamp(iotlb_default_size * 8, SIZE_512M, > > > > > > > + SIZE_512M); > > > > > > > + > > > > > > > + pr_info("SEV adjusted max SWIOTLB size = %luMB", > > > > > > > + size >> 20); > > > > > > > + } > > > > > > > + > > > > > > > + return size; > > > > > > > +} > > > > > > > + > > > > > > > void __init mem_encrypt_init(void) > > > > > > > { > > > > > > > if (!sme_me_mask) > > > > > > > diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h > > > > > > > index 046bb94bd4d6..01ae6d891327 100644 > > > > > > > --- a/include/linux/swiotlb.h > > > > > > > +++ b/include/linux/swiotlb.h > > > > > > > @@ -33,6 +33,7 @@ extern void swiotlb_init(int verbose); > > > > > > > int swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose); > > > > > > > extern unsigned long swiotlb_nr_tbl(void); > > > > > > > unsigned long swiotlb_size_or_default(void); > > > > > > > +extern void __init swiotlb_adjust(void); > > > > > > > extern int swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs); > > > > > > > extern void __init swiotlb_update_mem_attributes(void); > > > > > > > > > > > > > > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c > > > > > > > index c19379fabd20..66a9e627bb51 100644 > > > > > > > --- a/kernel/dma/swiotlb.c > > > > > > > +++ b/kernel/dma/swiotlb.c > > > > > > > @@ -163,6 +163,33 @@ unsigned long swiotlb_size_or_default(void) > > > > > > > return size ? size : (IO_TLB_DEFAULT_SIZE); > > > > > > > } > > > > > > > > > > > > > > +unsigned long __init __weak arch_swiotlb_adjust(unsigned long size) > > > > > > > +{ > > > > > > > + return 0; > > > > > > > +} > > > > > > > + > > > > > > > +void __init swiotlb_adjust(void) > > > > > > > +{ > > > > > > > + unsigned long size; > > > > > > > + > > > > > > > + /* > > > > > > > + * If swiotlb parameter has not been specified, give a chance to > > > > > > > + * architectures such as those supporting memory encryption to > > > > > > > + * adjust/expand SWIOTLB size for their use. > > > > > > > + */ > > > > > > > + if (!io_tlb_nslabs) { > > > > > > > + size = arch_swiotlb_adjust(IO_TLB_DEFAULT_SIZE); > > > > > > > + if (size) { > > > > > > > + size = ALIGN(size, 1 << IO_TLB_SHIFT); > > > > > > > + io_tlb_nslabs = size >> IO_TLB_SHIFT; > > > > > > > + io_tlb_nslabs = ALIGN(io_tlb_nslabs, IO_TLB_SEGSIZE); > > > > > > > + > > > > > > > + pr_info("architecture adjusted SWIOTLB slabs = %lu\n", > > > > > > > + io_tlb_nslabs); > > > > > > > + } > > > > > > > + } > > > > > > > +} > > > > > > > + > > > > > > > void swiotlb_print_info(void) > > > > > > > { > > > > > > > unsigned long bytes = io_tlb_nslabs << IO_TLB_SHIFT; > > > > > > > -- > > > > > > > 2.17.1 > > > > > > >
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