linux-kernel - Re: [PATCH v1.1 1/2] x86/sev: Use per-CPU PSC structure in prep for unaccepted memory support

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Date:   Wed, 3 Aug 2022 14:18:53 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v1.1 1/2] x86/sev: Use per-CPU PSC structure in prep for
 unaccepted memory support

On 8/3/22 14:03, Tom Lendacky wrote:
>> This whole iteration does look good to me versus the per-cpu version, so
>> I say go ahead with doing this for v2 once you wait a bit for any more
>> feedback.
> 
> I'm still concerned about the whole spinlock and performance. What if I
> reduce the number of entries in the PSC structure to, say, 64, which
> reduces the size of the struct to 520 bytes. Any issue if that is put on
> the stack, instead? It definitely makes things less complicated and
> feels like a good compromise on the size vs the number of PSC VMGEXIT
> requests.

That would be fine too.

But, I doubt there will be any real performance issues coming out of
this.  As bad as this MSR thing is, I suspect it's not half as
disastrous as the global spinlock in Kirill's patches.

Also, private<->shared page conversions are *NOT* common from what I can
tell.  There are a few pages converted at boot, but most host the
guest<->host communications are through the swiotlb pages which are static.

Are there other things that SEV uses this structure for that I'm missing?