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

Vitaly Kuznetsov
2021-04-22T09:51:09Z

Sean Christopherson <seanjc@google.com> 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@oracle.com>
> Cc: David Woodhouse <dwmw@amazon.co.uk>
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> 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@redhat.com>

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);

--
Vitaly