On 3/1/2024 7:07 AM, Sean Christopherson wrote:
> 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 :-)
Me too.
>>
>>> 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:
"intecerpted" -> "intercepted"
>>>
>>> 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" ?
> 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@intel.com>