[RFT PATCH v2 07/23] x86/sev: Separate MSR and GHCB based snp_cpuid() via a callback

Ard Biesheuvel
2025-05-04T09:53:20Z

From: Ard Biesheuvel <ardb@kernel.org>

There are two distinct callers of snp_cpuid(): one where the MSR
protocol is always used, and one where the GHCB page based interface is
always used.

The snp_cpuid() logic does not care about the distinction, which only
matters at a lower level. But the fact that it supports both interfaces
means that the GHCB page based logic is pulled into the early startup
code where PA to VA conversions are problematic, given that it runs from
the 1:1 mapping of memory.

So keep snp_cpuid() itself in the startup code, but factor out the
hypervisor calls via a callback, so that the GHCB page handling can be
moved out.

Code refactoring only - no functional change intended.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/boot/startup/sev-shared.c | 58 ++++----------------
arch/x86/coco/sev/vc-shared.c | 49 ++++++++++++++++-
arch/x86/include/asm/sev.h | 3 +-
3 files changed, 61 insertions(+), 49 deletions(-)

diff --git a/arch/x86/boot/startup/sev-shared.c b/arch/x86/boot/startup/sev-shared.c
index 7a706db87b93..408e064a80d9 100644
--- a/arch/x86/boot/startup/sev-shared.c
+++ b/arch/x86/boot/startup/sev-shared.c
@@ -342,44 +342,7 @@ static int __sev_cpuid_hv_msr(struct cpuid_leaf *leaf)
return ret;
}

-static int __sev_cpuid_hv_ghcb(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
-{
- u32 cr4 = native_read_cr4();
- int ret;
-
- ghcb_set_rax(ghcb, leaf->fn);
- ghcb_set_rcx(ghcb, leaf->subfn);
-
- if (cr4 & X86_CR4_OSXSAVE)
- /* Safe to read xcr0 */
- ghcb_set_xcr0(ghcb, xgetbv(XCR_XFEATURE_ENABLED_MASK));
- else
- /* xgetbv will cause #UD - use reset value for xcr0 */
- ghcb_set_xcr0(ghcb, 1);
-
- ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0);
- if (ret != ES_OK)
- return ret;
-
- if (!(ghcb_rax_is_valid(ghcb) &&
- ghcb_rbx_is_valid(ghcb) &&
- ghcb_rcx_is_valid(ghcb) &&
- ghcb_rdx_is_valid(ghcb)))
- return ES_VMM_ERROR;

- leaf->eax = ghcb->save.rax;
- leaf->ebx = ghcb->save.rbx;
- leaf->ecx = ghcb->save.rcx;
- leaf->edx = ghcb->save.rdx;
-
- return ES_OK;
-}
-
-static int sev_cpuid_hv(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
-{
- return ghcb ? __sev_cpuid_hv_ghcb(ghcb, ctxt, leaf)
- : __sev_cpuid_hv_msr(leaf);
-}

/*
* This may be called early while still running on the initial identity
@@ -484,21 +447,21 @@ snp_cpuid_get_validated_func(struct cpuid_leaf *leaf)
return false;
}

-static void snp_cpuid_hv(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
+static void snp_cpuid_hv_no_ghcb(void *ctx, struct cpuid_leaf *leaf)
{
- if (sev_cpuid_hv(ghcb, ctxt, leaf))
+ if (__sev_cpuid_hv_msr(leaf))
sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_CPUID_HV);
}

static int __head
-snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
- struct cpuid_leaf *leaf)
+snp_cpuid_postprocess(void (*cpuid_hv)(void *ctx, struct cpuid_leaf *),
+ void *ctx, struct cpuid_leaf *leaf)
{
struct cpuid_leaf leaf_hv = *leaf;

switch (leaf->fn) {
case 0x1:
- snp_cpuid_hv(ghcb, ctxt, &leaf_hv);
+ cpuid_hv(ctx, &leaf_hv);

/* initial APIC ID */
leaf->ebx = (leaf_hv.ebx & GENMASK(31, 24)) | (leaf->ebx & GENMASK(23, 0));
@@ -517,7 +480,7 @@ snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
break;
case 0xB:
leaf_hv.subfn = 0;
- snp_cpuid_hv(ghcb, ctxt, &leaf_hv);
+ cpuid_hv(ctx, &leaf_hv);

/* extended APIC ID */
leaf->edx = leaf_hv.edx;
@@ -565,7 +528,7 @@ snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
}
break;
case 0x8000001E:
- snp_cpuid_hv(ghcb, ctxt, &leaf_hv);
+ cpuid_hv(ctx, &leaf_hv);

/* extended APIC ID */
leaf->eax = leaf_hv.eax;
@@ -587,7 +550,8 @@ snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
* should be treated as fatal by caller.
*/
int __head
-snp_cpuid(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
+snp_cpuid(void (*cpuid_hv)(void *ctx, struct cpuid_leaf *),
+ void *ctx, struct cpuid_leaf *leaf)
{
const struct snp_cpuid_table *cpuid_table = snp_cpuid_get_table();

@@ -621,7 +585,7 @@ snp_cpuid(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
return 0;
}

- return snp_cpuid_postprocess(ghcb, ctxt, leaf);
+ return snp_cpuid_postprocess(cpuid_hv, ctx, leaf);
}

/*
@@ -648,7 +612,7 @@ void __head do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
leaf.fn = fn;
leaf.subfn = subfn;

- ret = snp_cpuid(NULL, NULL, &leaf);
+ ret = snp_cpuid(snp_cpuid_hv_no_ghcb, NULL, &leaf);
if (!ret)
goto cpuid_done;

diff --git a/arch/x86/coco/sev/vc-shared.c b/arch/x86/coco/sev/vc-shared.c
index 2c0ab0fdc060..b4688f69102e 100644
--- a/arch/x86/coco/sev/vc-shared.c
+++ b/arch/x86/coco/sev/vc-shared.c
@@ -409,15 +409,62 @@ static enum es_result vc_handle_ioio(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
return ret;
}

+static int __sev_cpuid_hv_ghcb(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
+{
+ u32 cr4 = native_read_cr4();
+ int ret;
+
+ ghcb_set_rax(ghcb, leaf->fn);
+ ghcb_set_rcx(ghcb, leaf->subfn);
+
+ if (cr4 & X86_CR4_OSXSAVE)
+ /* Safe to read xcr0 */
+ ghcb_set_xcr0(ghcb, xgetbv(XCR_XFEATURE_ENABLED_MASK));
+ else
+ /* xgetbv will cause #UD - use reset value for xcr0 */
+ ghcb_set_xcr0(ghcb, 1);
+
+ ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0);
+ if (ret != ES_OK)
+ return ret;
+
+ if (!(ghcb_rax_is_valid(ghcb) &&
+ ghcb_rbx_is_valid(ghcb) &&
+ ghcb_rcx_is_valid(ghcb) &&
+ ghcb_rdx_is_valid(ghcb)))
+ return ES_VMM_ERROR;
+
+ leaf->eax = ghcb->save.rax;
+ leaf->ebx = ghcb->save.rbx;
+ leaf->ecx = ghcb->save.rcx;
+ leaf->edx = ghcb->save.rdx;
+
+ return ES_OK;
+}
+
+struct cpuid_ctx {
+ struct ghcb *ghcb;
+ struct es_em_ctxt *ctxt;
+};
+
+static void snp_cpuid_hv_ghcb(void *p, struct cpuid_leaf *leaf)
+{
+ struct cpuid_ctx *ctx = p;
+
+ if (__sev_cpuid_hv_ghcb(ctx->ghcb, ctx->ctxt, leaf))
+ sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_CPUID_HV);
+}
+
static int vc_handle_cpuid_snp(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
{
+ struct cpuid_ctx ctx = { ghcb, ctxt };
struct pt_regs *regs = ctxt->regs;
struct cpuid_leaf leaf;
int ret;

leaf.fn = regs->ax;
leaf.subfn = regs->cx;
- ret = snp_cpuid(ghcb, ctxt, &leaf);
+ ret = snp_cpuid(snp_cpuid_hv_ghcb, &ctx, &leaf);
if (!ret) {
regs->ax = leaf.eax;
regs->bx = leaf.ebx;
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 6158893786d6..f50a606be4f1 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -533,7 +533,8 @@ struct cpuid_leaf {
u32 edx;
};

-int snp_cpuid(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf);
+int snp_cpuid(void (*cpuid_hv)(void *ctx, struct cpuid_leaf *),
+ void *ctx, struct cpuid_leaf *leaf);

void __noreturn sev_es_terminate(unsigned int set, unsigned int reason);
enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
--
2.49.0.906.g1f30a19c02-goog