Re: [PATCH RFC] kvm: emulate avx vmovdq

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 20, 2024, Keith Busch wrote:
> To test, I executed the following program against a qemu emulated pci
> device resource. Prior to this kernel patch, it would fail with
> 
>   traps: vmovdq[378] trap invalid opcode ip:4006b2 sp:7ffe2f5bb680 error:0 in vmovdq[6b2,400000+1000]
 
...

> +static const struct gprefix pfx_avx_0f_6f_0f_7f = {
> +	N, I(Avx | Aligned, em_mov), N, I(Avx | Unaligned, em_mov),
> +};
> +
> +static const struct opcode avx_0f_table[256] = {
> +	/* 0x00 - 0x5f */
> +	X16(N), X16(N), X16(N), X16(N), X16(N), X16(N),
> +	/* 0x60 - 0x6F */
> +	X8(N), X4(N), X2(N), N,
> +	GP(SrcMem | DstReg | ModRM | Mov, &pfx_avx_0f_6f_0f_7f),
> +	/* 0x70 - 0x7F */
> +	X8(N), X4(N), X2(N), N,
> +	GP(SrcReg | DstMem | ModRM | Mov, &pfx_avx_0f_6f_0f_7f),
> +	/* 0x80 - 0xFF */
> +	X16(N), X16(N), X16(N), X16(N), X16(N), X16(N), X16(N), X16(N),
> +};

Mostly as an FYI, we're likely going to run into more than just VMOVDQU sooner
rather than later.  E.g. gcc-13 with -march=x86-64-v3 (which per Vitaly is now
the default gcc behavior for some distros[*]) compiles this chunk from KVM
selftests' kvm_fixup_exception():

	regs->rip = regs->r11;
	regs->r9 = regs->vector;
	regs->r10 = regs->error_code;

intto this monstronsity (which is clever, but oof).

  405313:       c4 e1 f9 6e c8          vmovq  %rax,%xmm1
  405318:       48 89 68 08             mov    %rbp,0x8(%rax)
  40531c:       48 89 e8                mov    %rbp,%rax
  40531f:       c4 c3 f1 22 c4 01       vpinsrq $0x1,%r12,%xmm1,%xmm0
  405325:       49 89 6d 38             mov    %rbp,0x38(%r13)
  405329:       c5 fa 7f 45 00          vmovdqu %xmm0,0x0(%rbp)

I wouldn't be surprised if the same packing shenanigans get employed when generating
code for a struct overlay of emulated MMIO.

[*] https://lore.kernel.org/all/20240920154422.2890096-1-vkuznets@redhat.com