[PATCH v3 1/2] x86/sev: Put PSC struct on the stack in prep for unaccepted memory support

[Tom Lendacky]

In advance of providing support for unaccepted memory, switch from using
kmalloc() for allocating the Page State Change (PSC) structure to using a
local variable that lives on the stack. This is needed to avoid a possible
recursive call into set_pages_state() if the kmalloc() call requires
(more) memory to be accepted, which would result in a hang.

The current size of the PSC struct is 2,032 bytes. To make the struct more
stack friendly, reduce the number of PSC entries from 253 down to 64,
resulting in a size of 520 bytes. This is a nice compromise on struct size
and total PSC requests while still allowing parallel PSC operations across
vCPUs.

Also, since set_pages_state() uses the per-CPU GHCB, add a static variable
that indicates when per-CPU GHCBs are available. Until they are available,
the GHCB MSR protocol is used to perform page state changes. For APs, the
per-CPU GHCB is created before they are started and registered upon
startup, so this flag can be used globally for the BSP and APs instead of
creating a per-CPU flag.

If either the reduction in PSC entries or the use of the MSR protocol
until the per-CPU GHCBs are available, result in any kind of performance
issue (that is not seen at the moment), use of a larger static PSC struct,
with fallback to the smaller stack version, or use the boot GHCB prior to
per-CPU GHCBs, can be investigated.

Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
 arch/x86/include/asm/sev-common.h |  9 +++++++--
 arch/x86/kernel/sev.c             | 32 +++++++++++++++++++++++--------
 2 files changed, 31 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
index b8357d6ecd47..6c3d61c5f6a3 100644
--- a/arch/x86/include/asm/sev-common.h
+++ b/arch/x86/include/asm/sev-common.h
@@ -106,8 +106,13 @@ enum psc_op {
 #define GHCB_HV_FT_SNP			BIT_ULL(0)
 #define GHCB_HV_FT_SNP_AP_CREATION	BIT_ULL(1)
 
-/* SNP Page State Change NAE event */
-#define VMGEXIT_PSC_MAX_ENTRY		253
+/*
+ * SNP Page State Change NAE event
+ *   The VMGEXIT_PSC_MAX_ENTRY determines the size of the PSC structure,
+ *   which is a local variable (stack usage) in set_pages_state(). Do not
+ *   increase this value without evaluating the impact to stack usage.
+ */
+#define VMGEXIT_PSC_MAX_ENTRY		64
 
 struct psc_hdr {
 	u16 cur_entry;
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index c05f0124c410..40268ce97aad 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -66,6 +66,17 @@ static struct ghcb boot_ghcb_page __bss_decrypted __aligned(PAGE_SIZE);
  */
 static struct ghcb *boot_ghcb __section(".data");
 
+/*
+ * A flag used by set_pages_state() that indicates when the per-CPU GHCB has
+ * been created and registered and thus can be used instead of using the MSR
+ * protocol. The set_pages_state() function eventually invokes vmgexit_psc(),
+ * which only works with a per-CPU GHCB.
+ *
+ * For APs, the per-CPU GHCB is created before they are started and registered
+ * upon startup, so this flag can be used globally for the BSP and APs.
+ */
+static bool ghcb_percpu_ready __section(".data");
+
 /* Bitmap of SEV features supported by the hypervisor */
 static u64 sev_hv_features __ro_after_init;
 
@@ -660,7 +671,7 @@ static void pvalidate_pages(unsigned long vaddr, unsigned int npages, bool valid
 	}
 }
 
-static void __init early_set_pages_state(unsigned long paddr, unsigned int npages, enum psc_op op)
+static void early_set_pages_state(unsigned long paddr, unsigned int npages, enum psc_op op)
 {
 	unsigned long paddr_end;
 	u64 val;
@@ -751,6 +762,8 @@ static int vmgexit_psc(struct snp_psc_desc *desc)
 	unsigned long flags;
 	struct ghcb *ghcb;
 
+	WARN_ON_ONCE(!ghcb_percpu_ready);
+
 	/*
 	 * __sev_get_ghcb() needs to run with IRQs disabled because it is using
 	 * a per-CPU GHCB.
@@ -868,11 +881,14 @@ static void __set_pages_state(struct snp_psc_desc *data, unsigned long vaddr,
 static void set_pages_state(unsigned long vaddr, unsigned int npages, int op)
 {
 	unsigned long vaddr_end, next_vaddr;
-	struct snp_psc_desc *desc;
+	struct snp_psc_desc desc = {};
 
-	desc = kmalloc(sizeof(*desc), GFP_KERNEL_ACCOUNT);
-	if (!desc)
-		panic("SNP: failed to allocate memory for PSC descriptor\n");
+	/*
+	 * Use the MSR protocol when the per-CPU GHCBs are not yet registered,
+	 * since vmgexit_psc() uses the per-CPU GHCB.
+	 */
+	if (!ghcb_percpu_ready)
+		return early_set_pages_state(__pa(vaddr), npages, op);
 
 	vaddr = vaddr & PAGE_MASK;
 	vaddr_end = vaddr + (npages << PAGE_SHIFT);
@@ -882,12 +898,10 @@ static void set_pages_state(unsigned long vaddr, unsigned int npages, int op)
 		next_vaddr = min_t(unsigned long, vaddr_end,
 				   (VMGEXIT_PSC_MAX_ENTRY * PAGE_SIZE) + vaddr);
 
-		__set_pages_state(desc, vaddr, next_vaddr, op);
+		__set_pages_state(&desc, vaddr, next_vaddr, op);
 
 		vaddr = next_vaddr;
 	}
-
-	kfree(desc);
 }
 
 void snp_set_memory_shared(unsigned long vaddr, unsigned int npages)
@@ -1254,6 +1268,8 @@ void setup_ghcb(void)
 		if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
 			snp_register_per_cpu_ghcb();
 
+		ghcb_percpu_ready = true;
+
 		return;
 	}
 
-- 
2.36.1