Messages in this thread | | | Date | Wed, 10 Aug 2022 11:42:04 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address |
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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
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