On 4/7/21 12:34 PM, Brijesh Singh wrote: > > 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 ? GHCB GPA registration is only supported and required for SEV-SNP guests. The final version of the spec documents that and should be published within the next few days. Thanks, Tom > > >> >>> 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. >>
