Messages in this thread Patch in this message |  | | Date | Wed, 23 Nov 2022 14:17:19 -0800 | Subject | Re: [PATCH v7 11/20] x86/virt/tdx: Add placeholder to construct TDMRs to cover all TDX memory regions | From | Dave Hansen <> |
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On 11/20/22 16:26, Kai Huang wrote: > TDX provides increased levels of memory confidentiality and integrity. > This requires special hardware support for features like memory > encryption and storage of memory integrity checksums. Not all memory > satisfies these requirements. > > As a result, the TDX introduced the concept of a "Convertible Memory
s/the TDX introduced/TDX introduces/
> Region" (CMR). During boot, the firmware builds a list of all of the > memory ranges which can provide the TDX security guarantees. The list > of these ranges is available to the kernel by querying the TDX module. > > The TDX architecture needs additional metadata to record things like > which TD guest "owns" a given page of memory. This metadata essentially > serves as the 'struct page' for the TDX module. The space for this > metadata is not reserved by the hardware up front and must be allocated > by the kernel and given to the TDX module. > > Since this metadata consumes space, the VMM can choose whether or not to > allocate it for a given area of convertible memory. If it chooses not > to, the memory cannot receive TDX protections and can not be used by TDX > guests as private memory. > > For every memory region that the VMM wants to use as TDX memory, it sets > up a "TD Memory Region" (TDMR). Each TDMR represents a physically > contiguous convertible range and must also have its own physically > contiguous metadata table, referred to as a Physical Address Metadata > Table (PAMT), to track status for each page in the TDMR range. > > Unlike a CMR, each TDMR requires 1G granularity and alignment. To > support physical RAM areas that don't meet those strict requirements, > each TDMR permits a number of internal "reserved areas" which can be > placed over memory holes. If PAMT metadata is placed within a TDMR it > must be covered by one of these reserved areas. > > Let's summarize the concepts: > > CMR - Firmware-enumerated physical ranges that support TDX. CMRs are > 4K aligned. > TDMR - Physical address range which is chosen by the kernel to support > TDX. 1G granularity and alignment required. Each TDMR has > reserved areas where TDX memory holes and overlapping PAMTs can > be put into.
s/put into/represented/
> PAMT - Physically contiguous TDX metadata. One table for each page size > per TDMR. Roughly 1/256th of TDMR in size. 256G TDMR = ~1G > PAMT. > > As one step of initializing the TDX module, the kernel configures > TDX-usable memory regions by passing an array of TDMRs to the TDX module. > > Constructing the array of TDMRs consists below steps: > > 1) Create TDMRs to cover all memory regions that the TDX module can use;
Slight tweak:
1) Create TDMRs to cover all memory regions that the TDX module will use for TD memory
The TDX module "uses" more memory than strictly the TMDR's.
> 2) Allocate and set up PAMT for each TDMR; > 3) Set up reserved areas for each TDMR.
s/Set up/Designate/
> Add a placeholder to construct TDMRs to do the above steps after all > TDX memory regions are verified to be truly convertible. Always free > TDMRs at the end of the initialization (no matter successful or not) > as TDMRs are only used during the initialization.
The changelog here actually looks really good to me so far.
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c > index 32af86e31c47..26048c6b0170 100644 > --- a/arch/x86/virt/vmx/tdx/tdx.c > +++ b/arch/x86/virt/vmx/tdx/tdx.c > @@ -445,6 +445,63 @@ static int build_tdx_memory(void) > return ret; > } > > +/* Calculate the actual TDMR_INFO size */ > +static inline int cal_tdmr_size(void)
I think we can spare the bytes to add "culate" in the function name so we don't think these are California TDMRs.
> +{ > + int tdmr_sz; > + > + /* > + * The actual size of TDMR_INFO depends on the maximum number > + * of reserved areas. > + * > + * Note: for TDX1.0 the max_reserved_per_tdmr is 16, and > + * TDMR_INFO size is aligned up to 512-byte. Even it is > + * extended in the future, it would be insane if TDMR_INFO > + * becomes larger than 4K. The tdmr_sz here should never > + * overflow. > + */ > + tdmr_sz = sizeof(struct tdmr_info); > + tdmr_sz += sizeof(struct tdmr_reserved_area) * > + tdx_sysinfo.max_reserved_per_tdmr;
First, I think 'tdx_sysinfo' should probably be a local variable in init_tdx_module() and have its address passed in here. Having global variables always makes it more opaque about who is initializing it.
Second, if this code is making assumptions about 'max_reserved_per_tdmr', then let's actually add assertions or sanity checks. For instance:
if (tdx_sysinfo.max_reserved_per_tdmr > MAX_TDMRS) return -1;
or even:
if (tdmr_sz > PAGE_SIZE) return -1;
It does almost no good to just assert what the limits are in a comment.
> + /* > + * TDX requires each TDMR_INFO to be 512-byte aligned. Always > + * round up TDMR_INFO size to the 512-byte boundary. > + */
<sigh> More silly comments.
The place to document this is TDMR_INFO_ALIGNMENT. If anyone wants to know what the alignment is, exactly, they can look at the definition. They don't need to be told *TWICE* what TDMR_INFO_ALIGNMENT #defines to in one comment.
> + return ALIGN(tdmr_sz, TDMR_INFO_ALIGNMENT); > +} > + > +static struct tdmr_info *alloc_tdmr_array(int *array_sz) > +{ > + /* > + * TDX requires each TDMR_INFO to be 512-byte aligned. > + * Use alloc_pages_exact() to allocate all TDMRs at once. > + * Each TDMR_INFO will still be 512-byte aligned since > + * cal_tdmr_size() always returns 512-byte aligned size. > + */
OK, I think you're just trolling me now. Two *MORE* mentions of the 512-byte alignment?
> + *array_sz = cal_tdmr_size() * tdx_sysinfo.max_tdmrs; > + > + /* > + * Zero the buffer so 'struct tdmr_info::size' can be > + * used to determine whether a TDMR is valid. > + * > + * Note: for TDX1.0 the max_tdmrs is 64 and TDMR_INFO size > + * is 512-byte. Even they are extended in the future, it > + * would be insane if the total size exceeds 4MB. > + */ > + return alloc_pages_exact(*array_sz, GFP_KERNEL | __GFP_ZERO); > +}
This looks massively over complicated.
Get rid of this function entirely. Then create:
static int tdmr_array_size(void) { return tdmr_size_single() * tdx_sysinfo.max_tdmrs; }
The *caller* can do:
tdmr_array = alloc_pages_exact(tdmr_array_size(), GFP_KERNEL | __GFP_ZERO); if (!tdmr_array) { ...
Then the error path is:
free_pages_exact(tdmr_array, tdmr_array_size());
Then, there are no size pointers going back and forth. Easy peasy. I'm OK with a little arithmetic being repeated.
> +/* > + * Construct an array of TDMRs to cover all TDX memory ranges. > + * The actual number of TDMRs is kept to @tdmr_num. > + */ > +static int construct_tdmrs(struct tdmr_info *tdmr_array, int *tdmr_num) > +{ > + /* Return -EINVAL until constructing TDMRs is done */ > + return -EINVAL; > +} > + > /* > * Detect and initialize the TDX module. > * > @@ -454,6 +511,9 @@ static int build_tdx_memory(void) > */ > static int init_tdx_module(void) > { > + struct tdmr_info *tdmr_array; > + int tdmr_array_sz; > + int tdmr_num;
I tend to write these like:
"tdmr_num" is the number of *a* TDMR.
"nr_tdmrs" is the number of TDMRs.
> int ret; > > /* > @@ -506,11 +566,34 @@ static int init_tdx_module(void) > ret = build_tdx_memory(); > if (ret) > goto out; > + > + /* Prepare enough space to construct TDMRs */ > + tdmr_array = alloc_tdmr_array(&tdmr_array_sz); > + if (!tdmr_array) { > + ret = -ENOMEM; > + goto out_free_tdx_mem; > + } > + > + /* Construct TDMRs to cover all TDX memory ranges */ > + ret = construct_tdmrs(tdmr_array, &tdmr_num); > + if (ret) > + goto out_free_tdmrs; > + > /* > * Return -EINVAL until all steps of TDX module initialization > * process are done. > */ > ret = -EINVAL; > +out_free_tdmrs: > + /* > + * The array of TDMRs is freed no matter the initialization is > + * successful or not. They are not needed anymore after the > + * module initialization. > + */ > + free_pages_exact(tdmr_array, tdmr_array_sz); > +out_free_tdx_mem: > + if (ret) > + free_tdx_memory(); > out: > /* > * Memory hotplug checks the hot-added memory region against the > diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h > index 8e273756098c..a737f2b51474 100644 > --- a/arch/x86/virt/vmx/tdx/tdx.h > +++ b/arch/x86/virt/vmx/tdx/tdx.h > @@ -80,6 +80,29 @@ struct tdsysinfo_struct { > }; > } __packed __aligned(TDSYSINFO_STRUCT_ALIGNMENT); > > +struct tdmr_reserved_area { > + u64 offset; > + u64 size; > +} __packed; > + > +#define TDMR_INFO_ALIGNMENT 512 > + > +struct tdmr_info { > + u64 base; > + u64 size; > + u64 pamt_1g_base; > + u64 pamt_1g_size; > + u64 pamt_2m_base; > + u64 pamt_2m_size; > + u64 pamt_4k_base; > + u64 pamt_4k_size; > + /* > + * Actual number of reserved areas depends on > + * 'struct tdsysinfo_struct'::max_reserved_per_tdmr. > + */ > + struct tdmr_reserved_area reserved_areas[0]; > +} __packed __aligned(TDMR_INFO_ALIGNMENT); > + > /* > * Do not put any hardware-defined TDX structure representations below > * this comment!
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