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

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Message-ID: <877dkuhcl7.fsf@vitty.brq.redhat.com>
Date:   Thu, 22 Apr 2021 11:51:00 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     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>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 7/9] KVM: x86/xen: Drop RAX[63:32] when processing
 hypercall

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);

-- 
Vitaly