Re: [RFC PATCH] x86/insn: support decode MOVSXD instruction for MMIO

[Tom Lendacky <thomas.lendacky@amd.com>]

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.

Thanks,
Tom

>