Support early pvalidation in startup_64

The previous SNP VM boot is slow majorly due to the expensive PSP
command LAUNCH_UPDATE per 4K page. With this patch, launch_update only
need to validate memory pages used before startup_64 and optinally
choose to launch_update other memory pages.

The early pvalidate assumes the E820 table does not cover
invalid memory pages and then will do on-demand validation and extend
the table.

arch/x86/boot/compressed/head_64.S

@@ -345,6 +345,14 @@ SYM_CODE_START(startup_64)
 	cld
 	cli
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	/* pvalidate memory on demand if SNP is enabled. */
+	pushq	%rsi
+	movq    %rsi, %rdi
+	call 	pvalidate_for_startup_64
+	popq	%rsi
+#endif
+
 	/* Setup data segments. */
 	xorl	%eax, %eax
 	movl	%eax, %ds

arch/x86/boot/compressed/sev-snp.c

@@ -9,6 +9,7 @@
 #include "misc.h"
 #include "error.h"
 
+#include <asm/e820/types.h>
 #include <asm/msr-index.h>
 #include <asm/sev-snp.h>
 #include <asm/sev-es.h>
@@ -139,3 +140,62 @@ void sev_snp_register_ghcb(unsigned long paddr)
 	/* Restore the GHCB MSR value */
 	sev_es_wr_ghcb_msr(old);
 }
+
+static void extend_e820_on_demand(struct boot_e820_entry *e820_entry,
+				  u64 needed_ram_end)
+{
+	u64 end, paddr;
+	if (!e820_entry) {
+		return;
+	}
+	// Validated memory must be aligned by PAGE_SIZE
+	end = ALIGN(e820_entry->addr + e820_entry->size, PAGE_SIZE);
+	if (needed_ram_end > end && e820_entry->type == E820_TYPE_RAM) {
+		for (paddr = end; paddr < needed_ram_end; paddr += PAGE_SIZE) {
+			sev_snp_issue_pvalidate_page(paddr, true);
+		}
+		e820_entry->size = needed_ram_end - e820_entry->addr;
+	}
+}
+
+/*
+ * Explictly pvalidate needed pages for decompressing the kernel.
+ * The E820_TYPE_RAM entry includes only validated memory. The kernel
+ * expects that the RAM entry's addr is fixed while the entry size is to be
+ * extended to cover addresses to the start of next entry.
+ * The function increases the RAM entry size to cover all possilble memory
+ * addresses until init_size.
+ * For example,  init_end = 0x4000000,
+ * [RAM: 0x0 - 0x0],                       M[RAM: 0x0 - 0xa0000]
+ * [RSVD: 0xa0000 - 0x10000]                [RSVD: 0xa0000 - 0x10000]
+ * [ACPI: 0x10000 - 0x20000]      ==>       [ACPI: 0x10000 - 0x20000]
+ * [RSVD: 0x800000 - 0x900000]              [RSVD: 0x800000 - 0x900000]
+ * [RAM: 0x1000000 - 0x2000000]            M[RAM: 0x1000000 - 0x2001000]
+ * [RAM: 0x2001000 - 0x2007000]            M[RAM: 0x2001000 - 0x4000000]
+
+ Other RAM memory after init_end is pvalidated by ms_hyperv_init_platform
+ */
+__visible void pvalidate_for_startup_64(struct boot_params *boot_params)
+{
+	struct boot_e820_entry *e820_entry;
+	u64 init_end =
+		boot_params->hdr.pref_address + boot_params->hdr.init_size;
+	u64 needed_end;
+	u8 i, nr_entries = boot_params->e820_entries;
+	if (!sev_snp_enabled()) {
+		return;
+	}
+	for (i = 0; i < nr_entries; ++i) {
+		/* Pvalidate memory holes in e820 RAM entries. */
+		e820_entry = &boot_params->e820_table[i];
+		if (i < nr_entries - 1) {
+			needed_end = boot_params->e820_table[i + 1].addr;
+			if (needed_end < e820_entry->addr) {
+				error("e820 table is not sorted.");
+			}
+		} else {
+			needed_end = init_end;
+		}
+		extend_e820_on_demand(e820_entry, needed_end);
+	}
+}