Re: [RFC] KVM: x86: emulate movdqa

[Avi Kivity <avi@redhat.com>]

On 01/07/2012 12:26 PM, Stefan Hajnoczi wrote:
> An Ubuntu 9.10 Karmic Koala guest is unable to boot or install due to
> missing movdqa emulation:
>
> kvm_exit: reason EXCEPTION_NMI rip 0x7fef3e025a7b info 7fef3e799000 80000b0e
> kvm_page_fault: address 7fef3e799000 error_code f
> kvm_emulate_insn: 0:7fef3e025a7b: 66 0f 7f 07 (prot64)

Install udis86 (and udsi86-devel) and build trace-cmd; it will decode
the opcode for you.

>
> movdqa %xmm0,(%rdi)
>
> This patch adds movdqa to the emulator but does not implement #GP when
> the memory operand is unaligned to 16 bytes.  I'm not sure whether
> alignment checking can be implemented as an opcode .flag or if it needs
> to be done in em_movdqa().

It should actually be automatic when the Sse flag is present, since it's
the norm for almost all SSE instructions.  There should be a .flag to
override it for movdqu.

> A more fundamental question: why do we have to emulate this guest
> userspace SSE instruction in the first place?  This host machine lacks
> EPT but can't we service the page fault and then retry execution inside
> the guest?

Not when the target is mmio - there is no possible mapping.  With your
patch, is there a kvm_mmio trace right after the movdqa emulation?

>  
> +static int em_movdqa(struct x86_emulate_ctxt *ctxt)
> +{
> +	/* TODO alignment */
> +	memcpy(&ctxt->dst.vec_val, &ctxt->src.vec_val, ctxt->op_bytes);
> +	return X86EMUL_CONTINUE;
> +}

em_mov() should be adjusted to work here.

> +
>  static int em_movdqu(struct x86_emulate_ctxt *ctxt)
>  {
>  	memcpy(&ctxt->dst.vec_val, &ctxt->src.vec_val, ctxt->op_bytes);
> @@ -3115,7 +3122,7 @@ static struct opcode group11[] = {
>  };
>  
>  static struct gprefix pfx_0f_6f_0f_7f = {
> -	N, N, N, I(Sse, em_movdqu),
> +	N, I(Sse, em_movdqa), N, I(Sse, em_movdqu),
>  };
>  

Need the Mov flag too (I see it's missing for movdqu as well); otherwise
the emulator will RMW the destination.

Don't forget a unit test (for both the aligned and unaligned case).

-- 
error compiling committee.c: too many arguments to function