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

Peter Gonda
2022-03-10T14:54:58Z

()

On Mon, Mar 7, 2022 at 2:35 PM Brijesh Singh <brijesh.singh@amd.com> wrote:
>
> 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;

I couldn't get this patch set to boot and I found that I was setting
these XSAVE cpuid bits wrong. This took me a while to debug because
inside of handle_vc_boot_ghcb() this -EINVAL means we jump into the
halt loop, in addition the early_printk()s inside of that function
don't seem to be working for me but should the halt in
handle_vc_boot_ghcb() be replaced with an sev_es_terminate() or
something?

I am still working on why the early_printk()s in that function are not
working, it seems that they lead to a different halt. Have you tested
any of those error paths manually? For example if you set your CPUID
bits to explicitly fail here do you see the expected printks?

> +
> + 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
>