linux-kernel - Re: [PATCH v2 3/3] KVM: x86: Move #PF retry tracking variables into emulation context

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Message-ID: <20200219151644.GB15888@linux.intel.com>
Date:   Wed, 19 Feb 2020 07:16:44 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] KVM: x86: Move #PF retry tracking variables into
 emulation context

On Wed, Feb 19, 2020 at 09:13:00AM +0100, Paolo Bonzini wrote:
> On 19/02/20 00:03, Sean Christopherson wrote:
> > Move last_retry_eip and last_retry_addr into the emulation context as
> > they are specific to retrying an instruction after emulation failure.
> > 
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> I'm not sure about this, since it's not used by emulate.c.  The other
> two patches are good.

The easy solution to that is to move retry_instruction() into emulate.c.
That would also allow making x86_page_table_writing_insn() static.  All
other functions invoked from retry_instruction() are exposed via kvm_host.h.

Moving last_retry_* into the emulator context hopefully makes it more clear
that this code is the only user of the variables, e.g. last_retry_eip can't
be set by some other non-emulator flow.

	if (ctxt->eip == last_retry_eip && last_retry_addr == cr2)
		return false;

	vcpu->arch.last_retry_eip = ctxt->eip;
	vcpu->arch.last_retry_addr = cr2;

> > ---
> >  arch/x86/include/asm/kvm_emulate.h |  4 ++++
> >  arch/x86/include/asm/kvm_host.h    |  3 ---
> >  arch/x86/kvm/x86.c                 | 11 ++++++-----
> >  3 files changed, 10 insertions(+), 8 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> > index a4ef19a6e612..a26c8de414e8 100644
> > --- a/arch/x86/include/asm/kvm_emulate.h
> > +++ b/arch/x86/include/asm/kvm_emulate.h
> > @@ -315,6 +315,10 @@ struct x86_emulate_ctxt {
> >  	bool gpa_available;
> >  	gpa_t gpa_val;
> >  
> > +	/* Track EIP and CR2/GPA when retrying faulting instruction on #PF. */
> > +	unsigned long last_retry_eip;
> > +	unsigned long last_retry_addr;
> > +
> >  	/*
> >  	 * decode cache
> >  	 */
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 9c79c41eb5f6..6312ea32bb41 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -752,9 +752,6 @@ struct kvm_vcpu_arch {
> >  
> >  	cpumask_var_t wbinvd_dirty_mask;
> >  
> > -	unsigned long last_retry_eip;
> > -	unsigned long last_retry_addr;
> > -
> >  	struct {
> >  		bool halted;
> >  		gfn_t gfns[roundup_pow_of_two(ASYNC_PF_PER_VCPU)];
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index f88b72932c35..d19eb776f297 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6407,6 +6407,7 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
> >  
> >  	kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
> >  
> > +	/* last_retry_{eip,addr} are persistent and must not be init'd here. */
> >  	ctxt->gpa_available = false;
> >  	ctxt->eflags = kvm_get_rflags(vcpu);
> >  	ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
> > @@ -6557,8 +6558,8 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
> >  	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> >  	unsigned long last_retry_eip, last_retry_addr, gpa = cr2_or_gpa;
> >  
> > -	last_retry_eip = vcpu->arch.last_retry_eip;
> > -	last_retry_addr = vcpu->arch.last_retry_addr;
> > +	last_retry_eip = ctxt->last_retry_eip;
> > +	last_retry_addr = ctxt->last_retry_addr;
> >  
> >  	/*
> >  	 * If the emulation is caused by #PF and it is non-page_table
> > @@ -6573,7 +6574,7 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
> >  	 * and the address again, we can break out of the potential infinite
> >  	 * loop.
> >  	 */
> > -	vcpu->arch.last_retry_eip = vcpu->arch.last_retry_addr = 0;
> > +	ctxt->last_retry_eip = ctxt->last_retry_addr = 0;
> >  
> >  	if (!(emulation_type & EMULTYPE_ALLOW_RETRY_PF))
> >  		return false;
> > @@ -6588,8 +6589,8 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
> >  	if (ctxt->eip == last_retry_eip && last_retry_addr == cr2_or_gpa)
> >  		return false;
> >  
> > -	vcpu->arch.last_retry_eip = ctxt->eip;
> > -	vcpu->arch.last_retry_addr = cr2_or_gpa;
> > +	ctxt->last_retry_eip = ctxt->eip;
> > +	ctxt->last_retry_addr = cr2_or_gpa;
> >  
> >  	if (!vcpu->arch.mmu->direct_map)
> >  		gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL);
> > 
>