[PATCH v12 32/46] x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers

Brijesh Singh
2022-03-07T21:35:26Z

From: Michael Roth <michael.roth@amd.com>

CPUID instructions generate a #VC exception for SEV-ES/SEV-SNP guests,
for which early handlers are currently set up to handle. In the case
of SEV-SNP, guests can use a configurable location in guest memory
that has been pre-populated with a firmware-validated CPUID table to
look up the relevant CPUID values rather than requesting them from
hypervisor via a VMGEXIT. Add the various hooks in the #VC handlers to
allow CPUID instructions to be handled via the table. The code to
actually configure/enable the table will be added in a subsequent
commit.

Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
arch/x86/include/asm/sev-common.h | 2 +
arch/x86/kernel/sev-shared.c | 324 ++++++++++++++++++++++++++++++
2 files changed, 326 insertions(+)

diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
index e9b6815b3b3d..0759af9b1acf 100644
--- a/arch/x86/include/asm/sev-common.h
+++ b/arch/x86/include/asm/sev-common.h
@@ -152,6 +152,8 @@ struct snp_psc_desc {
#define GHCB_TERM_PSC 1 /* Page State Change failure */
#define GHCB_TERM_PVALIDATE 2 /* Pvalidate failure */
#define GHCB_TERM_NOT_VMPL0 3 /* SNP guest is not running at VMPL-0 */
+#define GHCB_TERM_CPUID 4 /* CPUID-validation failure */
+#define GHCB_TERM_CPUID_HV 5 /* CPUID failure during hypervisor fallback */

#define GHCB_RESP_CODE(v) ((v) & GHCB_MSR_INFO_MASK)

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index b4d5558c9d0a..0f1375164ff0 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -24,6 +24,36 @@ struct cpuid_leaf {
u32 edx;
};

+/*
+ * Individual entries of the SNP CPUID table, as defined by the SNP
+ * Firmware ABI, Revision 0.9, Section 7.1, Table 14.
+ */
+struct snp_cpuid_fn {
+ u32 eax_in;
+ u32 ecx_in;
+ u64 xcr0_in;
+ u64 xss_in;
+ u32 eax;
+ u32 ebx;
+ u32 ecx;
+ u32 edx;
+ u64 __reserved;
+} __packed;
+
+/*
+ * SNP CPUID table, as defined by the SNP Firmware ABI, Revision 0.9,
+ * Section 8.14.2.6. Also noted there is the SNP firmware-enforced limit
+ * of 64 entries per CPUID table.
+ */
+#define SNP_CPUID_COUNT_MAX 64
+
+struct snp_cpuid_table {
+ u32 count;
+ u32 __reserved1;
+ u64 __reserved2;
+ struct snp_cpuid_fn fn[SNP_CPUID_COUNT_MAX];
+} __packed;
+
/*
* Since feature negotiation related variables are set early in the boot
* process they must reside in the .data section so as not to be zeroed
@@ -33,6 +63,19 @@ struct cpuid_leaf {
*/
static u16 ghcb_version __ro_after_init;

+/* Copy of the SNP firmware's CPUID page. */
+static struct snp_cpuid_table cpuid_table_copy __ro_after_init;
+
+/*
+ * These will be initialized based on CPUID table so that non-present
+ * all-zero leaves (for sparse tables) can be differentiated from
+ * invalid/out-of-range leaves. This is needed since all-zero leaves
+ * still need to be post-processed.
+ */
+static u32 cpuid_std_range_max __ro_after_init;
+static u32 cpuid_hyp_range_max __ro_after_init;
+static u32 cpuid_ext_range_max __ro_after_init;
+
static bool __init sev_es_check_cpu_features(void)
{
if (!has_cpuflag(X86_FEATURE_RDRAND)) {
@@ -242,6 +285,252 @@ static int sev_cpuid_hv(struct cpuid_leaf *leaf)
return ret;
}

+/*
+ * This may be called early while still running on the initial identity
+ * mapping. Use RIP-relative addressing to obtain the correct address
+ * while running with the initial identity mapping as well as the
+ * switch-over to kernel virtual addresses later.
+ */
+static const struct snp_cpuid_table *snp_cpuid_get_table(void)
+{
+ void *ptr;
+
+ asm ("lea cpuid_table_copy(%%rip), %0"
+ : "=r" (ptr)
+ : "p" (&cpuid_table_copy));
+
+ return ptr;
+}
+
+/*
+ * The SNP Firmware ABI, Revision 0.9, Section 7.1, details the use of
+ * XCR0_IN and XSS_IN to encode multiple versions of 0xD subfunctions 0
+ * and 1 based on the corresponding features enabled by a particular
+ * combination of XCR0 and XSS registers so that a guest can look up the
+ * version corresponding to the features currently enabled in its XCR0/XSS
+ * registers. The only values that differ between these versions/table
+ * entries is the enabled XSAVE area size advertised via EBX.
+ *
+ * While hypervisors may choose to make use of this support, it is more
+ * robust/secure for a guest to simply find the entry corresponding to the
+ * base/legacy XSAVE area size (XCR0=1 or XCR0=3), and then calculate the
+ * XSAVE area size using subfunctions 2 through 64, as documented in APM
+ * Volume 3, Rev 3.31, Appendix E.3.8, which is what is done here.
+ *
+ * Since base/legacy XSAVE area size is documented as 0x240, use that value
+ * directly rather than relying on the base size in the CPUID table.
+ *
+ * Return: XSAVE area size on success, 0 otherwise.
+ */
+static u32 snp_cpuid_calc_xsave_size(u64 xfeatures_en, bool compacted)
+{
+ const struct snp_cpuid_table *cpuid_table = snp_cpuid_get_table();
+ u64 xfeatures_found = 0;
+ u32 xsave_size = 0x240;
+ int i;
+
+ for (i = 0; i < cpuid_table->count; i++) {
+ const struct snp_cpuid_fn *e = &cpuid_table->fn[i];
+
+ if (!(e->eax_in == 0xD && e->ecx_in > 1 && e->ecx_in < 64))
+ continue;
+ if (!(xfeatures_en & (BIT_ULL(e->ecx_in))))
+ continue;
+ if (xfeatures_found & (BIT_ULL(e->ecx_in)))
+ continue;
+
+ xfeatures_found |= (BIT_ULL(e->ecx_in));
+
+ if (compacted)
+ xsave_size += e->eax;
+ else
+ xsave_size = max(xsave_size, e->eax + e->ebx);
+ }
+
+ /*
+ * Either the guest set unsupported XCR0/XSS bits, or the corresponding
+ * entries in the CPUID table were not present. This is not a valid
+ * state to be in.
+ */
+ if (xfeatures_found != (xfeatures_en & GENMASK_ULL(63, 2)))
+ return 0;
+
+ return xsave_size;
+}
+
+static bool
+snp_cpuid_get_validated_func(struct cpuid_leaf *leaf)
+{
+ const struct snp_cpuid_table *cpuid_table = snp_cpuid_get_table();
+ int i;
+
+ for (i = 0; i < cpuid_table->count; i++) {
+ const struct snp_cpuid_fn *e = &cpuid_table->fn[i];
+
+ if (e->eax_in != leaf->fn)
+ continue;
+
+ if (cpuid_function_is_indexed(leaf->fn) && e->ecx_in != leaf->subfn)
+ continue;
+
+ /*
+ * For 0xD subfunctions 0 and 1, only use the entry corresponding
+ * to the base/legacy XSAVE area size (XCR0=1 or XCR0=3, XSS=0).
+ * See the comments above snp_cpuid_calc_xsave_size() for more
+ * details.
+ */
+ if (e->eax_in == 0xD && (e->ecx_in == 0 || e->ecx_in == 1))
+ if (!(e->xcr0_in == 1 || e->xcr0_in == 3) || e->xss_in)
+ continue;
+
+ leaf->eax = e->eax;
+ leaf->ebx = e->ebx;
+ leaf->ecx = e->ecx;
+ leaf->edx = e->edx;
+
+ return true;
+ }
+
+ return false;
+}
+
+static void snp_cpuid_hv(struct cpuid_leaf *leaf)
+{
+ if (sev_cpuid_hv(leaf))
+ sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_CPUID_HV);
+}
+
+static int snp_cpuid_postprocess(struct cpuid_leaf *leaf)
+{
+ struct cpuid_leaf leaf_hv = *leaf;
+
+ switch (leaf->fn) {
+ case 0x1:
+ snp_cpuid_hv(&leaf_hv);
+
+ /* initial APIC ID */
+ leaf->ebx = (leaf_hv.ebx & GENMASK(31, 24)) | (leaf->ebx & GENMASK(23, 0));
+ /* APIC enabled bit */
+ leaf->edx = (leaf_hv.edx & BIT(9)) | (leaf->edx & ~BIT(9));
+
+ /* OSXSAVE enabled bit */
+ if (native_read_cr4() & X86_CR4_OSXSAVE)
+ leaf->ecx |= BIT(27);
+ break;
+ case 0x7:
+ /* OSPKE enabled bit */
+ leaf->ecx &= ~BIT(4);
+ if (native_read_cr4() & X86_CR4_PKE)
+ leaf->ecx |= BIT(4);
+ break;
+ case 0xB:
+ leaf_hv.subfn = 0;
+ snp_cpuid_hv(&leaf_hv);
+
+ /* extended APIC ID */
+ leaf->edx = leaf_hv.edx;
+ break;
+ case 0xD: {
+ bool compacted = false;
+ u64 xcr0 = 1, xss = 0;
+ u32 xsave_size;
+
+ if (leaf->subfn != 0 && leaf->subfn != 1)
+ return 0;
+
+ if (native_read_cr4() & X86_CR4_OSXSAVE)
+ xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
+ 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;
+ }
+
+ /*
+ * The PPR and APM aren't clear on what size should be
+ * encoded in 0xD:0x1:EBX when compaction is not enabled
+ * by either XSAVEC (feature bit 1) or XSAVES (feature
+ * bit 3) since SNP-capable hardware has these feature
+ * bits fixed as 1. KVM sets it to 0 in this case, but
+ * to avoid this becoming an issue it's safer to simply
+ * treat this as unsupported for SNP guests.
+ */
+ if (!(leaf->eax & (BIT(1) | BIT(3))))
+ return -EINVAL;
+
+ compacted = true;
+ }
+
+ xsave_size = snp_cpuid_calc_xsave_size(xcr0 | xss, compacted);
+ if (!xsave_size)
+ return -EINVAL;
+
+ leaf->ebx = xsave_size;
+ }
+ break;
+ case 0x8000001E:
+ snp_cpuid_hv(&leaf_hv);
+
+ /* extended APIC ID */
+ leaf->eax = leaf_hv.eax;
+ /* compute ID */
+ leaf->ebx = (leaf->ebx & GENMASK(31, 8)) | (leaf_hv.ebx & GENMASK(7, 0));
+ /* node ID */
+ leaf->ecx = (leaf->ecx & GENMASK(31, 8)) | (leaf_hv.ecx & GENMASK(7, 0));
+ break;
+ default:
+ /* No fix-ups needed, use values as-is. */
+ break;
+ }
+
+ return 0;
+}
+
+/*
+ * Returns -EOPNOTSUPP if feature not enabled. Any other non-zero return value
+ * should be treated as fatal by caller.
+ */
+static int snp_cpuid(struct cpuid_leaf *leaf)
+{
+ const struct snp_cpuid_table *cpuid_table = snp_cpuid_get_table();
+
+ if (!cpuid_table->count)
+ return -EOPNOTSUPP;
+
+ if (!snp_cpuid_get_validated_func(leaf)) {
+ /*
+ * Some hypervisors will avoid keeping track of CPUID entries
+ * where all values are zero, since they can be handled the
+ * same as out-of-range values (all-zero). This is useful here
+ * as well as it allows virtually all guest configurations to
+ * work using a single SNP CPUID table.
+ *
+ * To allow for this, there is a need to distinguish between
+ * out-of-range entries and in-range zero entries, since the
+ * CPUID table entries are only a template that may need to be
+ * augmented with additional values for things like
+ * CPU-specific information during post-processing. So if it's
+ * not in the table, set the values to zero. Then, if they are
+ * within a valid CPUID range, proceed with post-processing
+ * using zeros as the initial values. Otherwise, skip
+ * post-processing and just return zeros immediately.
+ */
+ leaf->eax = leaf->ebx = leaf->ecx = leaf->edx = 0;
+
+ /* Skip post-processing for out-of-range zero leafs. */
+ if (!(leaf->fn <= cpuid_std_range_max ||
+ (leaf->fn >= 0x40000000 && leaf->fn <= cpuid_hyp_range_max) ||
+ (leaf->fn >= 0x80000000 && leaf->fn <= cpuid_ext_range_max)))
+ return 0;
+ }
+
+ return snp_cpuid_postprocess(leaf);
+}
+
/*
* Boot VC Handler - This is the first VC handler during boot, there is no GHCB
* page yet, so it only supports the MSR based communication with the
@@ -252,6 +541,7 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
unsigned int subfn = lower_bits(regs->cx, 32);
unsigned int fn = lower_bits(regs->ax, 32);
struct cpuid_leaf leaf;
+ int ret;

/* Only CPUID is supported via MSR protocol */
if (exit_code != SVM_EXIT_CPUID)
@@ -259,9 +549,18 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)

leaf.fn = fn;
leaf.subfn = subfn;
+
+ ret = snp_cpuid(&leaf);
+ if (!ret)
+ goto cpuid_done;
+
+ if (ret != -EOPNOTSUPP)
+ goto fail;
+
if (sev_cpuid_hv(&leaf))
goto fail;

+cpuid_done:
regs->ax = leaf.eax;
regs->bx = leaf.ebx;
regs->cx = leaf.ecx;
@@ -556,12 +855,37 @@ static enum es_result vc_handle_ioio(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
return ret;
}

+static int vc_handle_cpuid_snp(struct pt_regs *regs)
+{
+ struct cpuid_leaf leaf;
+ int ret;
+
+ leaf.fn = regs->ax;
+ leaf.subfn = regs->cx;
+ ret = snp_cpuid(&leaf);
+ if (!ret) {
+ regs->ax = leaf.eax;
+ regs->bx = leaf.ebx;
+ regs->cx = leaf.ecx;
+ regs->dx = leaf.edx;
+ }
+
+ return ret;
+}
+
static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
struct es_em_ctxt *ctxt)
{
struct pt_regs *regs = ctxt->regs;
u32 cr4 = native_read_cr4();
enum es_result ret;
+ int snp_cpuid_ret;
+
+ snp_cpuid_ret = vc_handle_cpuid_snp(regs);
+ if (!snp_cpuid_ret)
+ return ES_OK;
+ if (snp_cpuid_ret != -EOPNOTSUPP)
+ return ES_VMM_ERROR;

ghcb_set_rax(ghcb, regs->ax);
ghcb_set_rcx(ghcb, regs->cx);
--
2.25.1