linux-kernel - Re: [PATCH 06/16] KVM: x86/mmu: WARN if upper 32 bits of legacy #PF error code are non-zero

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Date: Thu, 29 Feb 2024 15:07:59 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Yan Zhao <yan.y.zhao@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Michael Roth <michael.roth@....com>, Yu Zhang <yu.c.zhang@...ux.intel.com>, 
	Chao Peng <chao.p.peng@...ux.intel.com>, Fuad Tabba <tabba@...gle.com>, 
	David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 06/16] KVM: x86/mmu: WARN if upper 32 bits of legacy #PF
 error code are non-zero

On Fri, Mar 01, 2024, Kai Huang wrote:
> 
> 
> On 28/02/2024 3:41 pm, Sean Christopherson wrote:
> > WARN if bits 63:32 are non-zero when handling an intercepted legacy #PF,
> 
> I found "legacy #PF" is a little bit confusing but I couldn't figure out a
> better name either :-)
> 
> > as the error code for #PF is limited to 32 bits (and in practice, 16 bits
> > on Intel CPUS).  This behavior is architectural, is part of KVM's ABI
> > (see kvm_vcpu_events.error_code), and is explicitly documented as being
> > preserved for intecerpted #PF in both the APM:
> > 
> >    The error code saved in EXITINFO1 is the same as would be pushed onto
> >    the stack by a non-intercepted #PF exception in protected mode.
> > 
> > and even more explicitly in the SDM as VMCS.VM_EXIT_INTR_ERROR_CODE is a
> > 32-bit field.
> > 
> > Simply drop the upper bits of hardware provides garbage, as spurious
> 
> "of" -> "if" ?
> 
> > information should do no harm (though in all likelihood hardware is buggy
> > and the kernel is doomed).
> > 
> > Handling all upper 32 bits in the #PF path will allow moving the sanity
> > check on synthetic checks from kvm_mmu_page_fault() to npf_interception(),
> > which in turn will allow deriving PFERR_PRIVATE_ACCESS from AMD's
> > PFERR_GUEST_ENC_MASK without running afoul of the sanity check.
> > 
> > Note, this also why Intel uses bit 15 for SGX (highest bit on Intel CPUs)
> 
> "this" -> "this is" ?
> 
> > and AMD uses bit 31 for RMP (highest bit on AMD CPUs); using the highest
> > bit minimizes the probability of a collision with the "other" vendor,
> > without needing to plumb more bits through microcode.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   arch/x86/kvm/mmu/mmu.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 7807bdcd87e8..5d892bd59c97 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4553,6 +4553,13 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
> >   	if (WARN_ON_ONCE(fault_address >> 32))
> >   		return -EFAULT;
> >   #endif
> > +	/*
> > +	 * Legacy #PF exception only have a 32-bit error code.  Simply drop the
> 
> "have" -> "has" ?

This one I'll fix by making "exception" plural.

Thanks much for the reviews!

> 
> > +	 * upper bits as KVM doesn't use them for #PF (because they are never
> > +	 * set), and to ensure there are no collisions with KVM-defined bits.
> > +	 */
> > +	if (WARN_ON_ONCE(error_code >> 32))
> > +		error_code = lower_32_bits(error_code);
> >   	vcpu->arch.l1tf_flush_l1d = true;
> >   	if (!flags) {
> Reviewed-by: Kai Huang <kai.huang@...el.com>