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    SubjectRe: [PATCH 06/16] KVM: x86/mmu: WARN if upper 32 bits of legacy #PF error code are non-zero
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    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@google.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" ?

    > + * 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@intel.com>

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