On 4/7/21 9:21 AM, Tom Lendacky wrote: > On 4/7/21 8:35 AM, Brijesh Singh wrote: >> On 4/7/21 6:16 AM, Borislav Petkov wrote: >>> On Tue, Apr 06, 2021 at 10:47:18AM -0500, Brijesh Singh wrote: >>>> Before the GHCB is established the caller does not need to save and >>>> restore MSRs. The page_state_change() uses the GHCB MSR protocol and it >>>> can be called before and after the GHCB is established hence I am saving >>>> and restoring GHCB MSRs. >>> I think you need to elaborate on that, maybe with an example. What the >>> other sites using the GHCB MSR currently do is: >>> >>> 1. request by writing it >>> 2. read the response >>> >>> None of them save and restore it. >>> >>> So why here? >> GHCB provides two ways to exit from the guest to the hypervisor. The MSR >> protocol and NAEs. The MSR protocol is generally used before the GHCB is >> established. After the GHCB is established the guests typically uses the >> NAEs. All of the current call sites uses the MSR protocol before the >> GHCB is established so they do not need to save and restore the GHCB. >> The GHCB is established on the first #VC - >> arch/x86/boot/compressed/sev-es.c early_setup_sev_es(). The GHCB page >> must a shared page: >> >> early_setup_sev_es() >> >> set_page_decrypted() >> >> sev_snp_set_page_shared() >> >> The sev_snp_set_page_shared() called before the GHCB is established. >> While exiting from the decompression the sev_es_shutdown_ghcb() is >> called to deinit the GHCB. >> >> sev_es_shutdown_ghcb() >> >> set_page_encrypted() >> >> sev_snp_set_page_private() >> >> Now that sev_snp_set_private() is called after the GHCB is established. > I believe the current SEV-ES code always sets the GHCB address in the GHCB > MSR before invoking VMGEXIT, so I think you're safe either way. Worth > testing at least. Ah, I didn;t realize that the sev_es_ghcb_hv_call() helper sets the GHCB MSR before invoking VMGEXIT. I should be able to drop the save and restore during the page state change. Thanks Tom.
