linux-kernel - Re: [PATCH 5/6] KVM: x86: Implement kvm_arch_vcpu_pre_fault

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Message-ID: <ZmoNAQmwIH5tigyv@google.com>
Date: Wed, 12 Jun 2024 14:02:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	isaku.yamahata@...el.com, binbin.wu@...ux.intel.com, 
	rick.p.edgecombe@...el.com
Subject: Re: [PATCH 5/6] KVM: x86: Implement kvm_arch_vcpu_pre_fault_memory()

On Mon, Apr 22, 2024, Xiaoyao Li wrote:
> On 4/19/2024 4:59 PM, Paolo Bonzini wrote:
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 10e90788b263..a045b23964c0 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4647,6 +4647,78 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> >   	return direct_page_fault(vcpu, fault);
> >   }
> > +static int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code,
> > +		     u8 *level)

Align parameters:

static int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code,
			    u8 *level)

> > +{
> > +	int r;
> > +
> > +	/* Restrict to TDP page fault. */

This is fairly obvious from the code, what might not be obvious is _why_.  I'm
also ok dropping the comment entirely, but it's easy enough to provide a hint to
the reader.

> > +	if (vcpu->arch.mmu->page_fault != kvm_tdp_page_fault)
> > +		return -EOPNOTSUPP;
> > +
> > +retry:
> > +	r = __kvm_mmu_do_page_fault(vcpu, gpa, error_code, true, NULL, level);
> > +	if (r < 0)
> > +		return r;
> > +
> > +	switch (r) {
> > +	case RET_PF_RETRY:
> > +		if (signal_pending(current))
> > +			return -EINTR;
> > +		cond_resched();
> > +		goto retry;

Rather than a goto+retry from inside a switch statement, what about:

	int r;

	/* 
	 * Pre-faulting a GPA is supported only non-nested TDP, as indirect
	 * MMUs map either GVAs or L2 GPAs, not L1 GPAs.
	 */
	if (vcpu->arch.mmu->page_fault != kvm_tdp_page_fault)
		return -EOPNOTSUPP;

	do {
		if (signal_pending(current))
			return -EINTR;

		cond_resched();

		r = kvm_mmu_do_page_fault(vcpu, gpa, error_code, true, NULL, level);
	} while (r == RET_PF_RETRY);

	switch (r) {
	case RET_PF_FIXED:
	case RET_PF_SPURIOUS:
		break;

	case RET_PF_EMULATE:
		return -ENOENT;

	case RET_PF_CONTINUE:
	case RET_PF_INVALID:
	case RET_PF_RETRY:
	default:
		WARN_ON_ONCE(r >= 0);
		return -EIO;
	}

	return 0;