Re: [PATCH v10 23/50] KVM: SEV: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

[Kalra, Ashish]

Hello Vlastimil,

On 12/11/2023 7:24 AM, Vlastimil Babka wrote:
> On 10/16/23 15:27, Michael Roth wrote:
> > From: Brijesh Singh <brijesh.singh@xxxxxxx>
> >
> > Implement a workaround for an SNP erratum where the CPU will incorrectly
> > signal an RMP violation #PF if a hugepage (2mb or 1gb) collides with the
> > RMP entry of a VMCB, VMSA or AVIC backing page.
> >
> > When SEV-SNP is globally enabled, the CPU marks the VMCB, VMSA, and AVIC
> > backing pages as "in-use" via a reserved bit in the corresponding RMP
> > entry after a successful VMRUN. This is done for _all_ VMs, not just
> > SNP-Active VMs.
> >
> > If the hypervisor accesses an in-use page through a writable
> > translation, the CPU will throw an RMP violation #PF. On early SNP
> > hardware, if an in-use page is 2mb aligned and software accesses any
> > part of the associated 2mb region with a hupage, the CPU will
> > incorrectly treat the entire 2mb region as in-use and signal a spurious
> > RMP violation #PF.
> >
> > The recommended is to not use the hugepage for the VMCB, VMSA or
> > AVIC backing page for similar reasons. Add a generic allocator that will
> > ensure that the page returns is not hugepage (2mb or 1gb) and is safe to
>
> This is a bit confusing wording as we are not avoiding "using a
> hugepage" but AFAIU, avoiding using a (4k) page that has a hugepage
> aligned physical address, right?

Yes.

> > be used when SEV-SNP is enabled. Also implement similar handling for the
> > VMCB/VMSA pages of nested guests.
> >
> > Co-developed-by: Marc Orr <marcorr@xxxxxxxxxx>
> > Signed-off-by: Marc Orr <marcorr@xxxxxxxxxx>
> > Reported-by: Alper Gun <alpergun@xxxxxxxxxx> # for nested VMSA case
> > Co-developed-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> > [mdr: squash in nested guest handling from Ashish]
> > Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> > ---
>
> <snip>
>
> > +
> > +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu)
> > +{
> > +	unsigned long pfn;
> > +	struct page *p;
> > +
> > +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> > +		return alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> > +
> > +	/*
> > +	 * Allocate an SNP safe page to workaround the SNP erratum where
> > +	 * the CPU will incorrectly signal an RMP violation  #PF if a
> > +	 * hugepage (2mb or 1gb) collides with the RMP entry of VMCB, VMSA
> > +	 * or AVIC backing page. The recommeded workaround is to not use the
> > +	 * hugepage.
>
> Same here "not use the hugepage"
>
> > +	 *
> > +	 * Allocate one extra page, use a page which is not 2mb aligned
> > +	 * and free the other.
>
> This makes more sense.
>
> > +	 */
> > +	p = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO, 1);
> > +	if (!p)
> > +		return NULL;
> > +
> > +	split_page(p, 1);
> > Yeah I think that's a sensible use of split_page(), as we don't have
> support for forcefully non-aligned allocations or specific "page
> coloring" in the page allocator.

Yes, using split_page() allows us to free the additionally allocated 
page individually.

Thanks,
Ashish

> So even with my wording concerns:
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>