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: Tue, 12 Mar 2024 13:44:04 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>, 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 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@...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>