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, 0 insertions, 13 deletions

diff --git a/arch/x86/boot/cpuflags.c b/arch/x86/boot/cpuflags.c
index 63e037e94e4c..916bac09b464 100644
--- a/arch/x86/boot/cpuflags.c
+++ b/arch/x86/boot/cpuflags.c
@@ -106,18 +106,5 @@ void get_cpuflags(void)
 			cpuid(0x80000001, &ignored, &ignored, &cpu.flags[6],
 			      &cpu.flags[1]);
 		}
-
-		if (max_amd_level >= 0x8000001f) {
-			u32 ebx;
-
-			/*
-			 * The X86_FEATURE_COHERENCY_SFW_NO feature bit is in
-			 * the virtualization flags entry (word 8) and set by
-			 * scattered.c, so the bit needs to be explicitly set.
-			 */
-			cpuid(0x8000001f, &ignored, &ebx, &ignored, &ignored);
-			if (ebx & BIT(31))
-				set_bit(X86_FEATURE_COHERENCY_SFW_NO, cpu.flags);
-		}
 	}
 }