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

Paolo Bonzini
2021-04-22T10:35:48Z

On 22/04/21 11:51, Vitaly Kuznetsov wrote:
> 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);
>

You haven't seen patch 9 yet. :)

Paolo