[PATCH v4] x86/virt/sev: Revert "Drop WBINVD before setting MSR_AMD64_SYSCFG_SNP_EN"

[Tycho Andersen <tycho@kernel.org>]

From: "Tycho Andersen (AMD)" <tycho@kernel.org>

This reverts commit 99cf1fb58e68 ("x86/virt/sev: Drop WBINVD before setting
MSR_AMD64_SYSCFG_SNP_EN").

Section 8.8 of the SNP spec says:

    Before invoking SNP_INIT_EX with INIT_RMP set to 1, software must
    ensure that no CPUs contain dirty cache lines for the memory containing
    the RMP.

Cachelines can be moved from cache to cache in a dirty state. The
wbinvd_on_all_cpus() before SNP_INIT_EX flushes the caches for each cpu,
but if the IPIs for WBINVD race with this dirty cacheline movement, it is
possible that they may not get flushed violating the firmware requirement.

Doing wbinvd_on_all_cpus() before setting SNPEn is safer since the RMP
table is not yet in use.

[ Heroically bisected by Srikanth. ]
Reported-by: Srikanth Aithal <Srikanth.Aithal@amd.com>
Tested-by: Srikanth Aithal <Srikanth.Aithal@amd.com>
Fixes: 99cf1fb58e68 ("x86/virt/sev: Drop WBINVD before setting MSR_AMD64_SYSCFG_SNP_EN")
Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
---
 arch/x86/virt/svm/sev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index 8bcdce98f6dc..cff285d8ad8e 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -536,6 +536,8 @@ int snp_prepare(void)
 		goto unlock;
 	}
 
+	wbinvd_on_all_cpus();
+
 	/*
 	 * MtrrFixDramModEn is not shared between threads on a core,
 	 * therefore it must be set on all CPUs prior to enabling SNP.

base-commit: aa6d9def48ea424a50c21de90ebe609c383cb05d
-- 
2.55.0