VMSA_GPA = 0xFFFFFFFFF000 is 48-bit, confused....

[JustPlay]

https://github.com/IBM/sev-snp-measure/blob/90f6e59831d20e44d03d5ee19388f624fca87291/sevsnpmeasure/gctx.py#L40

in the code, VMSA_GPA = 0xFFFFFFFFF000, but 0xFFFFFFFFF000 is 48-bits, not 52-bits

when launch sev-snp vm, we use cbitpos=51,reduced-phys-bits=1, so i think it use 51-bit (0-50) physical address space, with cbit=1, so should VMSA_GPA = 0xFFFFFFFFFF000 ?

[dubek]

I agree, this value is only 48-bits. I also see the same value in snp-digest-rs:

https://github.com/slp/snp-digest-rs/blob/0e5a787e99069944467151101ae4db474793d657/src/main.rs#L86

Note that with the current value I'm able to successfully verify launch digests of SNP guests, so the question is why the PSP is using this 48-bit value when filling the PAGE_INFO (SNP spec table 67).

Section 8.17.2.2 of the SNP spec says:

• GPA: The gPA of the 4 KB page. The firmware uses the RMP.GPA of the page.

Maybe the spec should be a bit clearer.

And in the host kernel, the RMP setting of the VMSA page is set with gpa = (u64)-1 (second argument in this call):

https://github.com/AMDESE/linux/blob/3a15da8647b99d147ed3ae8daf7cfdb19b20d83c/arch/x86/kvm/svm/sev.c#L2188

[tlendacky]

Section 8.17.2 provides some further detail. The input GPA must be page aligned (CMDBUF_SNP_LAUNCH_UPDATE structure, byte offset 0x10, bits 11:0 must be zero) and the second from the last paragraph of that section states "the guest physical address space is limited according to CPUID Fn80000008_EAX and thus the GPAs used by the firmware in measurement calculation are equally limited".

Given that information and that CPUID Fn80000008_EAX[PhysAddrSize]=0x30:
-1 == 0xFFFFFFFFFFFFFFFF => 0xFFFFFFFFF000 is used as the VMSA GPA during measuring.

I think the hypervisor patches should be made a bit clearer by performing this actual transformation of the supplied address.

[tlendacky]

Sorry, I confused the SPA and the GPA in the first part of that last response. The spec should probably be updated to clarify that the GPA will be page aligned and thus bits 11:0 will be zero.