[PATCH kernel] x86/compressed/64: reduce #VC nesting for intercepted CPUID for SEV-SNP guest

[Alexey Kardashevskiy]

For certain intercepts an SNP guest uses the GHCB protocol to talk to
the hypervisor from the #VC handler. The protocol requires a shared page so
there is one per vCPU. In case NMI arrives in a middle of #VC or the NMI
handler triggers a #VC, there is another "backup" GHCB page which stores
the content of the first one while SVM_VMGEXIT_NMI_COMPLETE is sent.
The vc_raw_handle_exception() handler manages main and backup GHCB pages
via __sev_get_ghcb/__sev_put_ghcb.

This works fine for #VC and occasional NMIs. This does not work so fine if
the #VC handler causes intercept + another #VC, if NMI arrives during
the second #VC, there are no more pages for SVM_VMGEXIT_NMI_COMPLETE.
The problem place is the #VC CPUID handler. Running perf in the SNP guest
crashes with:

Kernel panic - not syncing: Unable to handle #VC exception! GHCB and Backup GHCB are already in use

vc_raw_handle_exception #1: exit_code 72 (CPUID) eax d ecx 1
We lock the main GHCB and while it is locked we get to
snp_cpuid_postprocess() which executes "rdmsr" of MSR_IA32_XSS==0xda0 which
triggers:

vc_raw_handle_exception #2: exit_code 7c (MSR) ecx da0
Here we lock the backup ghcb.

And then PMC NMI comes which cannot complete as there is no GHCB page left
to use:

CPU: 5 PID: 566 Comm: touch Not tainted 6.5.0-rc2-aik-ad9c-g7413e71d3dcf-dirty #27
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown unknown
Call Trace:
 <NMI>
 dump_stack_lvl+0x44/0x60
 panic+0x222/0x310
 ____sev_get_ghcb+0x21e/0x220
 __sev_es_nmi_complete+0x28/0xf0
 exc_nmi+0x1ac/0x1c0
 end_repeat_nmi+0x16/0x67
...
 </NMI>
 <TASK>
 vc_raw_handle_exception+0x9e/0x2c0
 kernel_exc_vmm_communication+0x4d/0xa0
 asm_exc_vmm_communication+0x31/0x60
RIP: 0010:snp_cpuid+0x2ad/0x420

Drop one #VC by replacing "rdmsr" with GHCB's VMGEXIT to read the value from
the hypervisor.

Fixes: ee0bfa08a345 ("x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers")
Cc: x86@xxxxxxxxxx
Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
---

This is made on top of (which has the "efi/unaccepted: Make sure unaccepted table is mapped"
fix for booting SNP):
b996cbe1203c (tip/master) 15 hours ago Ingo Molnar Merge branch into tip/master: 'x86/tdx'

plus:
https://lore.kernel.org/lkml/a5856fa1ebe3879de91a8f6298b6bbd901c61881.1690578565.git.thomas.lendacky@xxxxxxx/
---
 arch/x86/kernel/sev-shared.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index e73c90c9cc5b..399219de5a9b 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -477,11 +477,19 @@ static int snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
 		if (leaf->subfn == 1) {
 			/* Get XSS value if XSAVES is enabled. */
 			if (leaf->eax & BIT(3)) {
-				unsigned long lo, hi;
-
-				asm volatile("rdmsr" : "=a" (lo), "=d" (hi)
-						     : "c" (MSR_IA32_XSS));
-				xss = (hi << 32) | lo;
+				/*
+				 * Since we're here, it is SNP and rdmsr will trigger
+				 * another #VC and waste one of just two GHCB pages.
+				 * Skip the intercept and do direct hypercall.
+				 */
+				ghcb_set_rcx(ghcb, MSR_IA32_XSS);
+				if (sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_MSR, 0, 0) != ES_OK)
+					return -EINVAL;
+
+				xss = (ghcb->save.rdx << 32) | ghcb->save.rax;
+
+				/* Invalidate qwords for likely another following GHCB call */
+				vc_ghcb_invalidate(ghcb);
 			}
 
 			/*
-- 
2.41.0