linux-kernel - Re: [PATCH] KVM: SVM: Mark SEV launch secret pages as dirty.

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Date:   Wed, 23 Sep 2020 10:26:46 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Cfir Cohen <cfir@...gle.com>,
        "kvm @ vger . kernel . org" <kvm@...r.kernel.org>,
        Lendacky Thomas <thomas.lendacky@....com>,
        Singh Brijesh <brijesh.singh@....com>,
        Grimm Jon <Jon.Grimm@....com>,
        David Rientjes <rientjes@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: Mark SEV launch secret pages as dirty.

On Wed, Sep 23, 2020 at 07:16:08PM +0200, Paolo Bonzini wrote:
> On 23/09/20 19:04, Sean Christopherson wrote:
> >> Two of the three instances are a bit different though.  What about this
> >> which at least shortens the comment to 2 fewer lines:
> > Any objection to changing those to "Flush (on non-coherent CPUs)"?  I agree
> > it would be helpful to call out the details, especially for DBG_*, but I
> > don't like that it reads as if the flush is unconditional.
> 
> Hmm... It's already fairly long lines so that would wrap to 3 lines, and

Dang, I was hoping it would squeeze into 2.

> the reference to the conditional flush wasn't there before either.

Well, the flush wasn't conditional before (ignoring the NULL check).
 
> sev_clflush_pages could be a better place to mention that (or perhaps
> it's self-explanatory).

I agree, but with

	/*
	 * Flush before LAUNCH_UPDATE encrypts pages in place, in case the cache
	 * contains the data that was written unencrypted.
 	 */
 	sev_clflush_pages(inpages, npages);

there's nothing in the comment or code that even suggests sev_clflush_pages() is
conditional, i.e. no reason for the reader to peek at the implemenation.

What about:

	/*
	 * Flush (on non-coherent CPUs) before LAUNCH_UPDATE encrypts pages in
	 * place, the cache may contain data that was written unencrypted.
	 */