x86/sev: Move GHCB page based HV communication out of startup code

Both the decompressor and the core kernel implement an early #VC handler,
which only deals with CPUID instructions, and full featured one, which can
handle any #VC exception.

The former communicates with the hypervisor using the MSR based protocol,
whereas the latter uses a shared GHCB page, which is configured a bit later
during the boot, when the kernel runs from its ordinary virtual mapping,
rather than the 1:1 mapping that the startup code uses.

Accessing this shared GHCB page from the core kernel's startup code is
problematic, because it involves converting the GHCB address provided by the
caller to a physical address. In the startup code, virtual to physical address
translations are problematic, given that the virtual address might be a 1:1
mapped address, and such translations should therefore be avoided.

This means that exposing startup code dealing with the GHCB to callers that
execute from the ordinary kernel virtual mapping should be avoided too. So
move all GHCB page based communication out of the startup code, now that all
communication occurring before the kernel virtual mapping is up relies on the
MSR protocol only.

As an exception, add a flag representing the need to apply the coherency
fix in order to avoid exporting CPUID* helpers because of the code
running too early for the *cpu_has* infrastructure.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-29-ardb+git@google.com

1 files changed, 2 insertions, 5 deletions

diff --git a/arch/x86/include/asm/sev-internal.h b/arch/x86/include/asm/sev-internal.h
index 3dfd306d1c9e..6199b35a82e4 100644
--- a/arch/x86/include/asm/sev-internal.h
+++ b/arch/x86/include/asm/sev-internal.h
@@ -2,7 +2,6 @@
 
 #define DR7_RESET_VALUE        0x400
 
-extern struct ghcb boot_ghcb_page;
 extern u64 sev_hv_features;
 extern u64 sev_secrets_pa;
 
@@ -80,7 +79,8 @@ static __always_inline u64 svsm_get_caa_pa(void)
 		return boot_svsm_caa_pa;
 }
 
-int svsm_perform_call_protocol(struct svsm_call *call);
+enum es_result verify_exception_info(struct ghcb *ghcb, struct es_em_ctxt *ctxt);
+void vc_forward_exception(struct es_em_ctxt *ctxt);
 
 static inline u64 sev_es_rd_ghcb_msr(void)
 {
@@ -97,9 +97,6 @@ static __always_inline void sev_es_wr_ghcb_msr(u64 val)
 	native_wrmsr(MSR_AMD64_SEV_ES_GHCB, low, high);
 }
 
-void snp_register_ghcb_early(unsigned long paddr);
-bool sev_es_negotiate_protocol(void);
-bool sev_es_check_cpu_features(void);
 u64 get_hv_features(void);
 
 const struct snp_cpuid_table *snp_cpuid_get_table(void);