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 17:17:05 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...el.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>,
        "Shutemov, Kirill" <kirill.shutemov@...el.com>,
        "Huang, Kai" <kai.huang@...el.com>
Subject: Re: [PATCH v1.1 1/2] x86/sev: Use per-CPU PSC structure in prep for
 unaccepted memory support

On 8/3/22 16:48, Dave Hansen wrote:
> On 8/3/22 14:34, Tom Lendacky wrote:
>>> 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.
>>
>> Generally, that's true. But, e.g., a dma_alloc_coherent() actually
>> doesn't go through SWIOTLB, but instead allocates the pages and makes
>> them shared, which results in a page state change. The NVMe driver was
>> calling that API a lot. In this case, though, the NVMe driver was
>> running in IRQ context and set_memory_decrypted() could sleep, so an
>> unencrypted DMA memory pool was created to work around the sleeping
>> issue and reduce the page state changes. It's just things like that,
>> that make me wary.
> 
> Interesting.  Is that a real passthrough NVMe device or the hypervisor
> presenting a virtual one that just happens to use the NVMe driver?

Hmmm... not sure, possibly the latter. I just knew that whatever it was, 
the NVMe driver was being used.

Thanks,
Tom

> 
> I'm pretty sure the TDX folks have been banking on having very few page
> state changes.  But, part of that at least is their expectation of
> relying heavily on virtio.
> 
> I wonder if their expectations are accurate, or if once TDX gets out
> into the real world if their hopes will be dashed.