[PATCH v6 14/42] x86/sev: Register GHCB memory when SEV-SNP is active

Brijesh Singh
2021-10-08T18:05:42Z

The SEV-SNP guest is required to perform GHCB GPA registration. This is
because the hypervisor may prefer that a guest use a consistent and/or
specific GPA for the GHCB associated with a vCPU. For more information,
see the GHCB specification section GHCB GPA Registration.

During the boot, init_ghcb() allocates a per-cpu GHCB page. On very first
VC exception, the exception handler switch to using the per-cpu GHCB page
allocated during the init_ghcb(). The GHCB page must be registered in
the current vcpu context.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
arch/x86/kernel/sev.c | 124 +++++++++++++++++++++++++-----------------
1 file changed, 75 insertions(+), 49 deletions(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 2290fbcc1844..4c891d5d9651 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -77,6 +77,13 @@ struct sev_es_runtime_data {
* is currently unsupported in SEV-ES guests.
*/
unsigned long dr7;
+
+ /*
+ * SEV-SNP requires that the GHCB must be registered before using it.
+ * The flag below will indicate whether the GHCB is registered, if its
+ * not registered then sev_es_get_ghcb() will perform the registration.
+ */
+ bool snp_ghcb_registered;
};

struct ghcb_state {
@@ -160,55 +167,6 @@ void noinstr __sev_es_ist_exit(void)
this_cpu_write(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC], *(unsigned long *)ist);
}

-/*
- * Nothing shall interrupt this code path while holding the per-CPU
- * GHCB. The backup GHCB is only for NMIs interrupting this path.
- *
- * Callers must disable local interrupts around it.
- */
-static noinstr struct ghcb *__sev_get_ghcb(struct ghcb_state *state)
-{
- struct sev_es_runtime_data *data;
- struct ghcb *ghcb;
-
- WARN_ON(!irqs_disabled());
-
- data = this_cpu_read(runtime_data);
- ghcb = &data->ghcb_page;
-
- if (unlikely(data->ghcb_active)) {
- /* GHCB is already in use - save its contents */
-
- if (unlikely(data->backup_ghcb_active)) {
- /*
- * Backup-GHCB is also already in use. There is no way
- * to continue here so just kill the machine. To make
- * panic() work, mark GHCBs inactive so that messages
- * can be printed out.
- */
- data->ghcb_active = false;
- data->backup_ghcb_active = false;
-
- instrumentation_begin();
- panic("Unable to handle #VC exception! GHCB and Backup GHCB are already in use");
- instrumentation_end();
- }
-
- /* Mark backup_ghcb active before writing to it */
- data->backup_ghcb_active = true;
-
- state->ghcb = &data->backup_ghcb;
-
- /* Backup GHCB content */
- *state->ghcb = *ghcb;
- } else {
- state->ghcb = NULL;
- data->ghcb_active = true;
- }
-
- return ghcb;
-}
-
/* Needed in vc_early_forward_exception */
void do_early_exception(struct pt_regs *regs, int trapnr);

@@ -464,6 +422,69 @@ static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt
/* Include code shared with pre-decompression boot stage */
#include "sev-shared.c"

+static void snp_register_ghcb(struct sev_es_runtime_data *data, unsigned long paddr)
+{
+ if (data->snp_ghcb_registered)
+ return;
+
+ snp_register_ghcb_early(paddr);
+
+ data->snp_ghcb_registered = true;
+}
+
+/*
+ * Nothing shall interrupt this code path while holding the per-CPU
+ * GHCB. The backup GHCB is only for NMIs interrupting this path.
+ *
+ * Callers must disable local interrupts around it.
+ */
+static noinstr struct ghcb *__sev_get_ghcb(struct ghcb_state *state)
+{
+ struct sev_es_runtime_data *data;
+ struct ghcb *ghcb;
+
+ WARN_ON(!irqs_disabled());
+
+ data = this_cpu_read(runtime_data);
+ ghcb = &data->ghcb_page;
+
+ if (unlikely(data->ghcb_active)) {
+ /* GHCB is already in use - save its contents */
+
+ if (unlikely(data->backup_ghcb_active)) {
+ /*
+ * Backup-GHCB is also already in use. There is no way
+ * to continue here so just kill the machine. To make
+ * panic() work, mark GHCBs inactive so that messages
+ * can be printed out.
+ */
+ data->ghcb_active = false;
+ data->backup_ghcb_active = false;
+
+ instrumentation_begin();
+ panic("Unable to handle #VC exception! GHCB and Backup GHCB are already in use");
+ instrumentation_end();
+ }
+
+ /* Mark backup_ghcb active before writing to it */
+ data->backup_ghcb_active = true;
+
+ state->ghcb = &data->backup_ghcb;
+
+ /* Backup GHCB content */
+ *state->ghcb = *ghcb;
+ } else {
+ state->ghcb = NULL;
+ data->ghcb_active = true;
+ }
+
+ /* SEV-SNP guest requires that GHCB must be registered. */
+ if (cc_platform_has(CC_ATTR_SEV_SNP))
+ snp_register_ghcb(data, __pa(ghcb));
+
+ return ghcb;
+}
+
static noinstr void __sev_put_ghcb(struct ghcb_state *state)
{
struct sev_es_runtime_data *data;
@@ -650,6 +671,10 @@ static bool __init setup_ghcb(void)
/* Alright - Make the boot-ghcb public */
boot_ghcb = &boot_ghcb_page;

+ /* SEV-SNP guest requires that GHCB GPA must be registered. */
+ if (cc_platform_has(CC_ATTR_SEV_SNP))
+ snp_register_ghcb_early(__pa(&boot_ghcb_page));
+
return true;
}

@@ -739,6 +764,7 @@ static void __init init_ghcb(int cpu)

data->ghcb_active = false;
data->backup_ghcb_active = false;
+ data->snp_ghcb_registered = false;
}

void __init sev_es_init_vc_handling(void)
--
2.25.1