linux-kernel - Re: [PATCH] KVM: x86: drop TSC offsetting kvm_x86_ops to fix KVM_GET/SET

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Date:   Wed, 2 Nov 2016 19:22:07 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        lcapitulino@...hat.com
Subject: Re: [PATCH] KVM: x86: drop TSC offsetting kvm_x86_ops to fix
 KVM_GET/SET_CLOCK

2016-11-02 09:38+0100, Paolo Bonzini:
> Since commit a545ab6a0085 ("kvm: x86: add tsc_offset field to struct
> kvm_vcpu_arch", 2016-09-07) the offset between host and L1 TSC is
> cached and need not be fished out of the VMCS or VMCB.  This means
> that we can implement adjust_tsc_offset_guest and read_l1_tsc
> entirely in generic code.  The simplification is particularly
> significant for VMX code, where vmx->nested.vmcs01_tsc_offset
> was duplicating what is now in vcpu->arch.tsc_offset.  Therefore
> the vmcs01_tsc_offset can be dropped completely.
> 
> More importantly, this fixes KVM_GET_CLOCK/KVM_SET_CLOCK
> which, after commit 108b249c453d ("KVM: x86: introduce get_kvmclock_ns",
> 2016-09-01) called read_l1_tsc while the VMCS was not loaded.
> It thus returned bogus values on Intel CPUs.
> 
> Fixes: 108b249c453dd7132599ab6dc7e435a7036c193f
> Reported-by: Roman Kagan <rkagan@...tuozzo.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  3 ---
>  arch/x86/kvm/svm.c              | 23 -----------------------
>  arch/x86/kvm/vmx.c              | 39 +++------------------------------------
>  arch/x86/kvm/x86.c              |  6 +++---
>  4 files changed, 6 insertions(+), 65 deletions(-)

Yay,

Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>