Re: [PATCH v1 2/2] x86/sev: Add SNP-specific unaccepted memory support

[Tom Lendacky]

On 8/22/22 19:24, Dionna Amalie Glaze wrote:
> > +void snp_accept_memory(phys_addr_t start, phys_addr_t end)
> > +{
> > +       unsigned long vaddr;
> > +       unsigned int npages;
> > +
> > +       if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
> > +               return;
> > +
> > +       vaddr = (unsigned long)__va(start);
> > +       npages = (end - start) >> PAGE_SHIFT;
> > +
> > +       set_pages_state(vaddr, npages, SNP_PAGE_STATE_PRIVATE);
> > +
> > +       pvalidate_pages(vaddr, npages, true);
> > +}
>
> My testing of this patch shows that a significant amount of time is
> spent using the MSR protocol to change page state, in such a
> significant fashion that it's slower than eagerly accepting all
> memory. The difference gets worse as the RAM size goes up, so I think
> there's some phase problem with the GHCB protocol not getting used
> early enough?

Thank you for testing. Let me see what I can find. I might have to rework 
Brijesh's original patches more to make use of the early boot GHCB in 
order to cut down on the number of MSR protocol requests.

Thanks,
Tom

>