linux-kernel - Re: [PATCH v7 02/11] KVM: pfncache: add a mark-dirty helper

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Message-ID: <734ac3e7-9fc4-47a4-9951-2fa04e10fe7d@xen.org>
Date:   Thu, 2 Nov 2023 17:52:02 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paul Durrant <pdurrant@...zon.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        David Woodhouse <dwmw2@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v7 02/11] KVM: pfncache: add a mark-dirty helper

On 31/10/2023 23:28, Sean Christopherson wrote:
> On Mon, Oct 02, 2023, Paul Durrant wrote:
>> From: Paul Durrant <pdurrant@...zon.com>
>>
>> At the moment pages are marked dirty by open-coded calls to
>> mark_page_dirty_in_slot(), directly deferefencing the gpa and memslot
>> from the cache. After a subsequent patch these may not always be set
>> so add a helper now so that caller will protected from the need to know
>> about this detail.
>>
>> NOTE: Pages are now marked dirty while the cache lock is held. This is
>>        to ensure that gpa and memslot are mutually consistent.
> 
> This absolutely belongs in a separate patch.  It sounds like a bug fix (haven't
> spent the time to figure out if it actually is), and even if it doesn't fix
> anything, burying something like this in a "add a helper" patch is just mean.
> 

Ok, I can split it out. It's a pretty minor fix so didn't seem worth it.

> 
>> diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
>> index 0f36acdf577f..b68ed7fa56a2 100644
>> --- a/virt/kvm/pfncache.c
>> +++ b/virt/kvm/pfncache.c
>> @@ -386,6 +386,12 @@ int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
>>   }
>>   EXPORT_SYMBOL_GPL(kvm_gpc_activate);
>>   
>> +void kvm_gpc_mark_dirty(struct gfn_to_pfn_cache *gpc)
>> +{
> 
> If there's actually a reason to call mark_page_dirty_in_slot() while holding @gpc's
> lock, then this should have a lockdep.  If there's no good reason, then don't move
> the invocation.
> 
>> +	mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpc->gpa >> PAGE_SHIFT);
>> +}
>> +EXPORT_SYMBOL_GPL(kvm_gpc_mark_dirty);
> 
> This doesn't need to be exported.  Hrm, none of the exports in this file are
> necessary, they likely all got added when we were thinking this stuff would be
> used for nVMX.  I think we should remove them, not because I'm worried about
> sub-modules doing bad things, but just because we should avoid polluting exported
> symbols as much as possible.

That in a separate clean-up patch too, I assume?

   Paul