On 4/24/24 06:54, Gerd Hoffmann wrote:
On Tue, Apr 23, 2024 at 03:59:58PM -0500, Michael Roth wrote:
For the most part, OVMF will clear the encryption bit for MMIO regions,
but there is currently one known exception during SEC when the APIC
base address is accessed via MMIO with the encryption bit set for
SEV-ES/SEV-SNP guests.
what exactly accesses the lapic that early?
InitializedApicTimer() in OvmfPkg/Sec/SecMain.c
+/**
+ Map known MMIO regions unencrypted if SEV-ES is active.
+
+ During early booting, page table entries default to having the encryption bit
+ set for SEV-ES/SEV-SNP guests. In cases where there is MMIO to an address,
the
+ encryption bit should be cleared. Clear it here for any known MMIO accesses
+ during SEC, which is currently just the APIC base address.
+
+**/
+VOID
+SecMapApicBaseUnencrypted (
+ VOID
+ )
+{
+ PAGE_MAP_AND_DIRECTORY_POINTER *Level4Entry;
+ PAGE_MAP_AND_DIRECTORY_POINTER *Level3Entry;
+ PAGE_MAP_AND_DIRECTORY_POINTER *Level2Entry;
+ PAGE_TABLE_4K_ENTRY *Level1Entry;
+ SEC_SEV_ES_WORK_AREA *SevEsWorkArea;
+ PHYSICAL_ADDRESS Cr3;
+ UINT64 ApicAddress;
+ UINT64 PgTableMask;
+ UINT32 Level1Page;
+ UINT64 Level1Address;
+ UINT64 Level1Flags;
+ UINTN PteIndex;
+
+ if (!SevEsIsEnabled ()) {
+ return;
+ }
That is incompatible with 5-level paging. The current reset vector will
never turn on 5-level paging in case SEV is active because we have more
incompatibilities elsewhere (BaseMemEncryptSevLib IIRC). But still,
it's moving things into the wrong direction ...
Agreed. SEV needs to clean up the pagetable manipulation in general in
order to support 5-level paging and remove redundant code. That will be
a patch series in itself.
But without this modification, the SNP support no longer works with the
KVM/gmem support that will be upstream. This change gets OVMF SNP
support working again.
Ideally CpuPageTableLib should be used for this.
CpuPageTableLib will need to be modified in order for it to be used at
this (Sec) stage. In order to work in Sec - either the caller will have
to supply a list of pages that can be used if pagetable entries need to
be allocated for splits or new entries or by providing some kind of SEC
pagetable allocation pool.
So it will take significant work to get SEV support updated to using
CpuPageTableLib and that's why with this single patch we can get OVMF
SNP support working again.
Thanks,
Tom
take care,
Gerd
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