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

[Tom Lendacky]

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.