On Wed, Aug 10, 2022 at 03:59:34AM +0000, Kalra, Ashish wrote:
> Actually SNP feature can be enabled globally, but SNP is activated on a per VM basis.
>
> From the APM:
> The term SNP-enabled indicates that SEV-SNP is globally enabled in the SYSCFG
> MSR. The term SNP-active indicates that SEV-SNP is enabled for a specific VM in the
> SEV_FEATURES field of its VMSA
Aha, and I was wondering whether "globally" meant the RMP needs to cover
all physical memory but I guess that isn't the case:
"RMP-Covered: Checks that the target page is covered by the RMP. A page
is covered by the RMP if its corresponding RMP entry is below RMP_END.
Any page not covered by the RMP is considered a Hypervisor-Owned page."
> >You need to elaborate more here: a RMP fault can happen and then the
> >page can get unmapped? What is the exact scenario here?
>
> Yes, if the page gets unmapped while the RMP fault was being handled,
> will add more explanation here.
So what's the logic here to return 1, i.e., retry?
Why should a fault for a page that gets unmapped be retried? The fault
in that case should be ignored, IMO. It'll have the same effect to
return from do_user_addr_fault() there, without splitting but you need
to have a separate return value definition so that it is clear what
needs to happen. And that return value should be != 0 so that the
current check still works.
> Actually, the above computes an index into the RMP table.
What index in the RMP table?
> It is basically an index into the 4K page within the hugepage mapped
> in the RMP table or in other words an index into the RMP table entry
> for 4K page(s) corresponding to a hugepage.
So pte_index(address) and for 1G pages, pmd_index(address).
So no reinventing the wheel if we already have helpers for that.
> It is mainly a wrapper around__split_huge_pmd() for SNP use case where
> the host hugepage is split to be in sync with the RMP table.
I see what it is. And I'm saying this looks wrong. You're enforcing page
splitting to be a valid thing to do only for SEV machines. Why?
Why is
if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
return VM_FAULT_SIGBUS;
there at all?
This is generic code you're touching - not arch/x86/.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
