linux-kernel - Re: [PATCH 13/15] KVM: SVM: Add and use svm_register_cache

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Date:   Thu, 18 Nov 2021 16:37:38 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 13/15] KVM: SVM: Add and use svm_register_cache_reset()

On 11/8/21 13:44, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> It resets all the appropriate bits like vmx.
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
>   arch/x86/kvm/svm/svm.c |  3 +--
>   arch/x86/kvm/svm/svm.h | 26 ++++++++++++++++++++++++++
>   2 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index b7da66935e72..ba9cfddd2875 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3969,8 +3969,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>   
>   	svm->vmcb->control.tlb_ctl = TLB_CONTROL_DO_NOTHING;
>   	vmcb_mark_all_clean(svm->vmcb);
> -
> -	kvm_register_clear_available(vcpu, VCPU_EXREG_PDPTR);
> +	svm_register_cache_reset(vcpu);
>   
>   	/*
>   	 * We need to handle MC intercepts here before the vcpu has a chance to
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 0d7bbe548ac3..1cf5d5e2d0cd 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -274,6 +274,32 @@ static inline bool vmcb_is_dirty(struct vmcb *vmcb, int bit)
>           return !test_bit(bit, (unsigned long *)&vmcb->control.clean);
>   }
>   
> +static inline void svm_register_cache_reset(struct kvm_vcpu *vcpu)
> +{
> +/*
> + * SVM_REGS_AVAIL_SET - The set of registers that will be updated in cache on
> + *			demand.  Other registers not listed here are synced to
> + *			the cache immediately after VM-Exit.
> + *
> + * SVM_REGS_DIRTY_SET - The set of registers that might be outdated in
> + *			architecture. Other registers not listed here are synced
> + *			to the architecture immediately when modifying.
> + *
> + *			Special case: VCPU_EXREG_CR3 should be in this set due
> + *			to the fact.  But KVM_REQ_LOAD_MMU_PGD is always
> + *			requested when the cache vcpu->arch.cr3 is changed and
> + *			svm_load_mmu_pgd() always syncs the new CR3 value into
> + *			the architecture.  So the dirty information of
> + *			VCPU_EXREG_CR3 is not used which means VCPU_EXREG_CR3
> + *			isn't required to be put in this set.
> + */
> +#define SVM_REGS_AVAIL_SET	(1 << VCPU_EXREG_PDPTR)
> +#define SVM_REGS_DIRTY_SET	(0)
> +
> +	vcpu->arch.regs_avail &= ~SVM_REGS_AVAIL_SET;
> +	vcpu->arch.regs_dirty &= ~SVM_REGS_DIRTY_SET;
> +}

I think touching regs_dirty is confusing here, so I'd go with this:

         vcpu->arch.regs_avail &= ~SVM_REGS_LAZY_LOAD_SET;

         /*
          * SVM does not use vcpu->arch.regs_dirty.  The only register that
          * might be out of date in the VMCB is CR3, but KVM_REQ_LOAD_MMU_PGD
          * is always requested when the cache vcpu->arch.cr3 is changed and
          * svm_load_mmu_pgd() always syncs the new CR3 value into the VMCB.
          */

(VMX instead needs VCPU_EXREG_CR3 mostly because it does not want to
update it unconditionally on exit).

Paolo