linux-kernel - Re: [PATCH] KVM: x86/mmu: use helper macro SPTE_ENT_PER

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Message-ID: <ab27ae74-b23a-9c23-a3c6-3328b0aa0f2f@huawei.com>
Date:   Sat, 15 Oct 2022 10:39:30 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <pbonzini@...hat.com>, <tglx@...utronix.de>, <mingo@...hat.com>,
        <bp@...en8.de>, <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
        <hpa@...or.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: use helper macro SPTE_ENT_PER_PAGE

On 2022/10/6 7:17, Sean Christopherson wrote:
> On Tue, Sep 13, 2022, Miaohe Lin wrote:
>> Use helper macro SPTE_ENT_PER_PAGE to get the number of spte entries
>> per page. Minor readability improvement.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
> 
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>

Many thanks for your review and comment. And sorry for late respond. A really busy week. :)

> 
>>  arch/x86/kvm/mmu/mmu.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index 858bc53cfab4..45c532d00f78 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -1645,7 +1645,7 @@ static int is_empty_shadow_page(u64 *spt)
>>  	u64 *pos;
>>  	u64 *end;
>>  
>> -	for (pos = spt, end = pos + PAGE_SIZE / sizeof(u64); pos != end; pos++)
>> +	for (pos = spt, end = pos + SPTE_ENT_PER_PAGE; pos != end; pos++)
> 
> This is buried under MMU_DEBUG, and turning that on to compile test, which requires
> manually changing kernel code to enable, results in some minor warnings.  Given the
> existence of CONFIG_KVM_WERROR=y, I think it's safe to say this code hasn't been
> exercised in a very long time.  E.g. this is literally the first time I've actually
> enabled MMU_DEBUG.
> 
> This particular check seems like it would be quite useful, but the pgprintk() and
> rmap_printk() hooks, not so much.  E.g. the knob is too coarse grained, and many
> of the prints now have tracepoints.
> 
> So, unless someone actually actively uses MMU_DEBUG+dbg, I'm inclined to just delete
> pgprintk() and rmap_printk(), and then rename MMU_WARN_ON => KVM_MMU_WARN_ON and
> add a Kconfig for that, e.g. CONFIG_KVM_PROVE_MMU.
> 
> Hmm, and maybe clean up this helper too, e.g. get rid of the pointer arithmetic,
> and take the full kvm_mmu_page so that the error message can print out things like
> the gfn (a host kernel pointer is going to be useless for debug).
> 
> Thoughts?  Objections?

I tend to agree with you. MMU_DEBUG might even be an obsolete DEBUG component. I believe above
changes will improve the current code. Am I supposed to do this or will you kindly do this?

Thanks again,
Miaohe Lin