[PATCH v4 2/4] x86/virt: Force GIF=1 prior to disabling SVM (for reboot flows)

Sean Christopherson
2022-11-30T23:37:09Z

Set GIF=1 prior to disabling SVM to ensure that INIT is recognized if the
kernel is disabling SVM in an emergency, e.g. if the kernel is about to
jump into a crash kernel or may reboot without doing a full CPU RESET.
If GIF is left cleared, the new kernel (or firmware) will be unabled to
awaken APs. Eat faults on STGI (due to EFER.SVME=0) as it's possible
that SVM could be disabled via NMI shootdown between reading EFER.SVME
and executing STGI.

Link: https://lore.kernel.org/all/cbcb6f35-e5d7-c1c9-4db9-fe5cc4de579a@amd.com
Cc: stable@vger.kernel.org
Cc: Andrew Cooper <Andrew.Cooper3@citrix.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/virtext.h | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/virtext.h b/arch/x86/include/asm/virtext.h
index 8757078d4442..0acb14806a74 100644
--- a/arch/x86/include/asm/virtext.h
+++ b/arch/x86/include/asm/virtext.h
@@ -126,7 +126,18 @@ static inline void cpu_svm_disable(void)

wrmsrl(MSR_VM_HSAVE_PA, 0);
rdmsrl(MSR_EFER, efer);
- wrmsrl(MSR_EFER, efer & ~EFER_SVME);
+ if (efer & EFER_SVME) {
+ /*
+ * Force GIF=1 prior to disabling SVM, e.g. to ensure INIT and
+ * NMI aren't blocked. Eat faults on STGI, as it #UDs if SVM
+ * isn't enabled and SVM can be disabled by an NMI callback.
+ */
+ asm_volatile_goto("1: stgi\n\t"
+ _ASM_EXTABLE(1b, %l[fault])
+ ::: "memory" : fault);
+fault:
+ wrmsrl(MSR_EFER, efer & ~EFER_SVME);
+ }
}

/** Makes sure SVM is disabled, if it is supported on the CPU
--
2.38.1.584.g0f3c55d4c2-goog