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

[Kirill A. Shutemov]

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@xxxxxxxxxxxxxxx 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?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov