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

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Message-ID: <874kfyh9vp.fsf@vitty.brq.redhat.com>
Date:   Thu, 22 Apr 2021 12:49:30 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Babu Moger <babu.moger@....com>,
        Joao Martins <joao.m.martins@...cle.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v2 7/9] KVM: x86/xen: Drop RAX[63:32] when processing
 hypercall

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 22/04/21 11:51, Vitaly Kuznetsov wrote:
>> Sean Christopherson <seanjc@...gle.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@...cle.com>
>>> Cc: David Woodhouse <dwmw@...zon.co.uk>
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: Sean Christopherson <seanjc@...gle.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@...hat.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