On 4/7/21 6:59 AM, Borislav Petkov wrote: > On Wed, Mar 24, 2021 at 11:44:18AM -0500, Brijesh Singh wrote: >> The SEV-SNP guest is required to perform GHCB GPA registration. This is > Why does it need to do that? Some additional security so as to not allow > changing the GHCB once it is established? > > I'm guessing that's enforced by the SNP fw and we cannot do that > retroactively for SEV...? Because it sounds like a nice little thing we > could do additionally. The feature is part of the GHCB version 2 and is enforced by the hypervisor. I guess it can be extended for the ES. Since this feature was not available in GHCB version 1 (base ES) so it should be presented as an optional for the ES ? > >> because the hypervisor may prefer that a guest use a consistent and/or >> specific GPA for the GHCB associated with a vCPU. For more information, >> see the GHCB specification section 2.5.2. > I think you mean > > "2.3.2 GHCB GPA Registration" > > Please use the section name too because that doc changes from time to > time. > > Also, you probably should update it here: > > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.kernel.org%2Fshow_bug.cgi%3Fid%3D206537&data=04%7C01%7Cbrijesh.singh%40amd.com%7Ce8ae7574ecc742be6c1a08d8f9bcac94%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637533936070042328%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=NaHJ5R9Dfo%2FPnci%2B%2B6xK9ecpV0%2F%2FYbsdGl25%2BFj3TaU%3D&reserved=0 > Yes, the section may have changed since I wrote the description. Noted. I will refer the section name. >> diff --git a/arch/x86/boot/compressed/sev-snp.c b/arch/x86/boot/compressed/sev-snp.c >> index 5c25103b0df1..a4c5e85699a7 100644 >> --- a/arch/x86/boot/compressed/sev-snp.c >> +++ b/arch/x86/boot/compressed/sev-snp.c >> @@ -113,3 +113,29 @@ void sev_snp_set_page_shared(unsigned long paddr) >> { >> sev_snp_set_page_private_shared(paddr, SNP_PAGE_STATE_SHARED); >> } >> + >> +void sev_snp_register_ghcb(unsigned long paddr) > Right and let's prefix SNP-specific functions with "snp_" only so that > it is clear which is wcich when looking at the code. > >> +{ >> + u64 pfn = paddr >> PAGE_SHIFT; >> + u64 old, val; >> + >> + if (!sev_snp_enabled()) >> + return; >> + >> + /* save the old GHCB MSR */ >> + old = sev_es_rd_ghcb_msr(); >> + >> + /* Issue VMGEXIT */ > No need for that comment. > >> + sev_es_wr_ghcb_msr(GHCB_REGISTER_GPA_REQ_VAL(pfn)); >> + VMGEXIT(); >> + >> + val = sev_es_rd_ghcb_msr(); >> + >> + /* If the response GPA is not ours then abort the guest */ >> + if ((GHCB_SEV_GHCB_RESP_CODE(val) != GHCB_REGISTER_GPA_RESP) || >> + (GHCB_REGISTER_GPA_RESP_VAL(val) != pfn)) >> + sev_es_terminate(GHCB_SEV_ES_REASON_GENERAL_REQUEST); > Yet another example where using a specific termination reason could help > with debugging guests. Looking at the GHCB spec, I hope GHCBData[23:16] > is big enough for all reasons. I'm sure it can be extended ofc ... Maybe we can request the GHCB version 3 to add the extended error code. > :-) > >> + /* Restore the GHCB MSR value */ >> + sev_es_wr_ghcb_msr(old); >> +} >> diff --git a/arch/x86/include/asm/sev-snp.h b/arch/x86/include/asm/sev-snp.h >> index f514dad276f2..0523eb21abd7 100644 >> --- a/arch/x86/include/asm/sev-snp.h >> +++ b/arch/x86/include/asm/sev-snp.h >> @@ -56,6 +56,13 @@ struct __packed snp_page_state_change { >> struct snp_page_state_entry entry[SNP_PAGE_STATE_CHANGE_MAX_ENTRY]; >> }; >> >> +/* GHCB GPA register */ >> +#define GHCB_REGISTER_GPA_REQ 0x012UL >> +#define GHCB_REGISTER_GPA_REQ_VAL(v) (GHCB_REGISTER_GPA_REQ | ((v) << 12)) >> + >> +#define GHCB_REGISTER_GPA_RESP 0x013UL > Let's append "UL" to the other request numbers for consistency. > > Thx. >
