Re: [PATCH v2 7/9] KVM: x86/xen: Drop RAX[63:32] when processing hypercall

[Paolo Bonzini]

On 22/04/21 11:51, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
>
> > Truncate RAX to 32 bits, i.e. consume EAX, when retrieving the hypecall
> > index for a Xen hypercall.  Per Xen documentation[*], the index is EAX
> > when the vCPU is not in 64-bit mode.
> >
> > [*] http://xenbits.xenproject.org/docs/sphinx-unstable/guest-guide/x86/hypercall-abi.html
> >
> > Fixes: 23200b7a30de ("KVM: x86/xen: intercept xen hypercalls if enabled")
> > Cc: Joao Martins <joao.m.martins@xxxxxxxxxx>
> > Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/xen.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> > index ae17250e1efe..7f27bb65a572 100644
> > --- a/arch/x86/kvm/xen.c
> > +++ b/arch/x86/kvm/xen.c
> > @@ -673,7 +673,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
> >   	bool longmode;
> >   	u64 input, params[6];
> >   
> > -	input = (u64)kvm_register_read(vcpu, VCPU_REGS_RAX);
> > +	input = (u64)kvm_register_readl(vcpu, VCPU_REGS_RAX);
> >   
> >   	/* Hyper-V hypercalls get bit 31 set in EAX */
> >   	if ((input & 0x80000000) &&
>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>
> Alternatively, as a minor optimization, you could've used '!longmode'
> check below, something like:
>
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index ae17250e1efe..7df1498d3a41 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -682,6 +682,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
>   
>          longmode = is_64_bit_mode(vcpu);
>          if (!longmode) {
> +               input = (u32)input;
>                  params[0] = (u32)kvm_rbx_read(vcpu);
>                  params[1] = (u32)kvm_rcx_read(vcpu);
>                  params[2] = (u32)kvm_rdx_read(vcpu);

You haven't seen patch 9 yet. :)

Paolo