Messages in this thread | | | Date | Fri, 31 Mar 2023 10:50:34 -0500 | Subject | Re: [RFC PATCH] x86/insn: support decode MOVSXD instruction for MMIO | From | Tom Lendacky <> |
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On 3/31/23 10:25, Kirill A. Shutemov wrote: > On Fri, Mar 31, 2023 at 09:33:31AM -0500, Tom Lendacky wrote: >> >> >> On 3/31/23 09:09, Kirill A. Shutemov wrote: >>> On Fri, Mar 31, 2023 at 08:40:30AM -0500, Tom Lendacky wrote: >>>> On 3/31/23 05:06, Kirill A. Shutemov wrote: >>>>> On Fri, Mar 31, 2023 at 08:49:48AM +0000, David Laight wrote: >>>>>> From: Wu Zongyong >>>>>>> Sent: 31 March 2023 03:24 >>>>>>> >>>>>>> On Thu, Mar 30, 2023 at 03:39:51PM +0300, kirill.shutemov@linux.intel.com wrote: >>>>>>>> On Wed, Mar 29, 2023 at 10:59:37AM +0800, Wu Zongyong wrote: >>>>>>>>> It seems MOVSXD which opcode is 0x63 is not handled, support >>>>>>>>> to decode it in insn_decode_mmio(). >>>>>>>> >>>>>>>> Do you have a particular user in mind? >>>>>>> To be honest, I don't find a specific user which uses the MOVSXD. >>>>>>> >>>>>>> But both Intel and AMD's instructions reference contains MOVSXD and lots >>>>>>> of MOVSXD instructions occur when I "objdump -S vmlinux", so I think it >>>>>>> may be useful to support it in insn_decode_mmio(). >>>>>>> >>>>>>> Are there some special consideration about this instruction? >>>>>> >>>>>> It is a sign-extending memory read (32bit to 64bit). >>>>>> You pretty much never want to do that to a device register. >>>>>> Also kernel code should be using readl() (etc) which do >>>>>> unsigned reads. >>>>>> So they should never happen for mmio. >>>>>> >>>>>> Of course, if you mmap() PCIe space directly into a program's >>>>>> address space anything might happen ... >>>>> >>>>> There are two users of the interface: TDX and SEV. TDX doesn't allow >>>>> userspace MMIO. SEV *seems* allows it, but I am not sure how it is safe. >>>>> >>>>> Tom? >>>> >>>> The insn_decode_mmio() function is only called by the SEV/TDX related code >>>> and is specifically MMIO oriented. As David said, this instruction is likely >>>> not being used for that in the kernel. If we come across a case where this >>>> is used, we can look at how it is being used in that situation and it can be >>>> addressed then. >>> >>> I was asking if SEV supports userspace MMIO. And if yes, how do you make >>> it safe? >>> >> >> No, SEV doesn't support userspace MMIO. > > But where do you filter out userspace MMIO? AFAICS, it goes straight from > from #VC to insn_decode_mmio(). Hm?
The userspace mapping would have the encryption bit set and MMIO to encrypted memory is detected and not allowed.
Thanks, Tom
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