[PATCH 17/62] x86/boot/compressed/64: Add function to map a page unencrypted

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

This function is needed to map the GHCB for SEV-ES guests. The GHCB is
used for communication with the hypervisor, so its content must not be
encrypted.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/boot/compressed/ident_map_64.c | 125 ++++++++++++++++++++++++
 arch/x86/boot/compressed/misc.h         |   1 +
 2 files changed, 126 insertions(+)

diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
index feb180cced28..04a5ff4bda66 100644
--- a/arch/x86/boot/compressed/ident_map_64.c
+++ b/arch/x86/boot/compressed/ident_map_64.c
@@ -26,6 +26,7 @@
 #include <asm/init.h>
 #include <asm/pgtable.h>
 #include <asm/trap_defs.h>
+#include <asm/cmpxchg.h>
 /* Use the static base for this part of the boot process */
 #undef __PAGE_OFFSET
 #define __PAGE_OFFSET __PAGE_OFFSET_BASE
@@ -157,6 +158,130 @@ void initialize_identity_maps(void)
 	write_cr3(top_level_pgt);
 }
 
+static pte_t *split_large_pmd(struct x86_mapping_info *info,
+			      pmd_t *pmdp, unsigned long __address)
+{
+	unsigned long page_flags;
+	unsigned long address;
+	pte_t *pte;
+	pmd_t pmd;
+	int i;
+
+	pte = (pte_t *)info->alloc_pgt_page(info->context);
+	if (!pte)
+		return NULL;
+
+	address     = __address & PMD_MASK;
+	/* No large page - clear PSE flag */
+	page_flags  = info->page_flag & ~_PAGE_PSE;
+
+	/* Populate the PTEs */
+	for (i = 0; i < PTRS_PER_PMD; i++) {
+		set_pte(&pte[i], __pte(address | page_flags));
+		address += PAGE_SIZE;
+	}
+
+	/*
+	 * Ideally we need to clear the large PMD first and do a TLB
+	 * flush before we write the new PMD. But the 2M range of the
+	 * PMD might contain the code we execute and/or the stack
+	 * we are on, so we can't do that. But that should be safe here
+	 * because we are going from large to small mappings and we are
+	 * also the only user of the page-table, so there is no chance
+	 * of a TLB multihit.
+	 */
+	pmd = __pmd((unsigned long)pte | info->kernpg_flag);
+	set_pmd(pmdp, pmd);
+	/* Flush TLB to establish the new PMD */
+	write_cr3(top_level_pgt);
+
+	return pte + pte_index(__address);
+}
+
+static void clflush_page(unsigned long address)
+{
+	unsigned int flush_size;
+	char *cl, *start, *end;
+
+	/*
+	 * Hardcode cl-size to 64 - CPUID can't be used here because that might
+	 * cause another #VC exception and the GHCB is not ready to use yet.
+	 */
+	flush_size = 64;
+	start      = (char *)(address & PAGE_MASK);
+	end        = start + PAGE_SIZE;
+
+	/*
+	 * First make sure there are no pending writes on the cache-lines to
+	 * flush.
+	 */
+	asm volatile("mfence" : : : "memory");
+
+	for (cl = start; cl != end; cl += flush_size)
+		clflush(cl);
+}
+
+static int __set_page_decrypted(struct x86_mapping_info *info,
+				unsigned long address)
+{
+	unsigned long scratch, *target;
+	pgd_t *pgdp = (pgd_t *)top_level_pgt;
+	p4d_t *p4dp;
+	pud_t *pudp;
+	pmd_t *pmdp;
+	pte_t *ptep, pte;
+
+	/*
+	 * First make sure there is a PMD mapping for 'address'.
+	 * It should already exist, but keep things generic.
+	 *
+	 * To map the page just read from it and fault it in if there is no
+	 * mapping yet. add_identity_map() can't be called here because that
+	 * would unconditionally map the address on PMD level, destroying any
+	 * PTE-level mappings that might already exist.  Also do something
+	 * useless with 'scratch' so the access won't be optimized away.
+	 */
+	target = (unsigned long *)address;
+	scratch = *target;
+	arch_cmpxchg(target, scratch, scratch);
+
+	/*
+	 * The page is mapped at least with PMD size - so skip checks and walk
+	 * directly to the PMD.
+	 */
+	p4dp = p4d_offset(pgdp, address);
+	pudp = pud_offset(p4dp, address);
+	pmdp = pmd_offset(pudp, address);
+
+	if (pmd_large(*pmdp))
+		ptep = split_large_pmd(info, pmdp, address);
+	else
+		ptep = pte_offset_kernel(pmdp, address);
+
+	if (!ptep)
+		return -ENOMEM;
+
+	/* Clear encryption flag and write new pte */
+	pte = pte_clear_flags(*ptep, _PAGE_ENC);
+	set_pte(ptep, pte);
+
+	/* Flush TLB to map the page unencrypted */
+	write_cr3(top_level_pgt);
+
+	/*
+	 * Changing encryption attributes of a page requires to flush it from
+	 * the caches.
+	 */
+	clflush_page(address);
+
+	return 0;
+}
+
+int set_page_decrypted(unsigned long address)
+{
+	return __set_page_decrypted(&mapping_info, address);
+}
+
 static void pf_error(unsigned long error_code, unsigned long address,
 		     struct pt_regs *regs)
 {
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 0e3508c5c15c..42f68a858a35 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -98,6 +98,7 @@ static inline void choose_random_location(unsigned long input,
 #endif
 
 #ifdef CONFIG_X86_64
+extern int set_page_decrypted(unsigned long address);
 extern unsigned char _pgtable[];
 #endif
 
-- 
2.17.1