x86/sev: Put PSC struct on the stack in prep for unaccepted memory support

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.

If the reduction in PSC entries results 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, can be investigated.

For more background info on this decision, see the subthread in the Link:
tag below.
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/lkml/658c455c40e8950cb046dd885dd19dc1c52d060a.1659103274.git.thomas.lendacky@amd.com

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 stack variable 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;

arch/x86/kernel/sev.c

@@ -882,11 +882,7 @@ static void __set_pages_state(struct snp_psc_desc *data, unsigned long vaddr,
 static void set_pages_state(unsigned long vaddr, unsigned long npages, int op)
 {
 	unsigned long vaddr_end, next_vaddr;
-	struct snp_psc_desc *desc;
-
-	desc = kmalloc(sizeof(*desc), GFP_KERNEL_ACCOUNT);
-	if (!desc)
-		panic("SNP: failed to allocate memory for PSC descriptor\n");
+	struct snp_psc_desc desc;
 
 	vaddr = vaddr & PAGE_MASK;
 	vaddr_end = vaddr + (npages << PAGE_SHIFT);
@@ -896,12 +892,10 @@ static void set_pages_state(unsigned long vaddr, unsigned long 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 long npages)