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

Vitaly Kuznetsov
2021-04-22T10:49:39Z

Paolo Bonzini <pbonzini@redhat.com> writes:

> 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. :)
>

True; suggestion dismissed :-)

--
Vitaly