linux-kernel - Re: [PATCH 04/17] KVM: x86: Move synthetic PFERR

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Date: Wed, 15 May 2024 09:03:32 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, Kai Huang <kai.huang@...el.com>,
 Binbin Wu <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH 04/17] KVM: x86: Move synthetic PFERR_* sanity checks to
 SVM's #NPF handler

On 5/14/2024 11:32 PM, Sean Christopherson wrote:
> On Tue, May 14, 2024, Xiaoyao Li wrote:
>> On 5/14/2024 1:31 AM, Sean Christopherson wrote:
>>> On Mon, May 13, 2024, Xiaoyao Li wrote:
>>>> On 5/7/2024 11:58 PM, Paolo Bonzini wrote:
>>>>> +#define PFERR_SYNTHETIC_MASK	(PFERR_IMPLICIT_ACCESS)
>>>>>     #define PFERR_NESTED_GUEST_PAGE (PFERR_GUEST_PAGE_MASK |	\
>>>>>     				 PFERR_WRITE_MASK |		\
>>>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>>>> index c72a2033ca96..5562d693880a 100644
>>>>> --- a/arch/x86/kvm/mmu/mmu.c
>>>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>>>> @@ -4502,6 +4502,9 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
>>>>>     		return -EFAULT;
>>>>>     #endif
>>>>> +	/* Ensure the above sanity check also covers KVM-defined flags. */
>>>>
>>>> 1. There is no sanity check above related to KVM-defined flags yet. It has
>>>> to be after Patch 6.
>>>
>>> Ya, it's not just the comment, the entire changelog expects this patch to land
>>> after patch 6.
>>>>
>>>> 2. I somehow cannot parse the comment properly, though I know it's to ensure
>>>> KVM-defined PFERR_SYNTHETIC_MASK not contain any bit below 32-bits.
>>>
>>> Hmm, how about this?
>>>
>>> 	/*
>>> 	 * Ensure that the above sanity check on hardware error code bits 63:32
>>> 	 * also prevents false positives on KVM-defined flags.
>>> 	 */
>>>
>>
>> Maybe it's just myself inability, I still cannot interpret it well.
>>
>> Can't we put it above the sanity check of error code, and just with a
>> comment like
>>
>> 	/*
>>   	 * Ensure KVM-defined flags not occupied any bits below 32-bits,
>>         * that are used by hardware.
> 
> This is somewhat misleading, as hardware does use bits 63:32 (for #NPF), just not
> for #PF error codes.  And the reason I'm using rather indirect wording is that
> KVM _could_ define synthetic flags in bits 31:0, there's simply a higher probability
> of needing to reshuffle bit numbers due to a conflict with a future feature.
> 
> Is this better?  I think it captures what you're looking for, while hopefully also
> capturing that staying out of bits 31:0 isn't a hard requirement.

yeah, it looks better!

> 	/*
> 	 * Restrict KVM-defined flags to bits 63:32 so that it's impossible for
> 	 * them to conflict with #PF error codes, which are limited to 32 bits.
> 	 */