linux-kernel - [PATCH 1/5] x86/coco: Use slow_virt_to_phys() in page transition hypervisor callbacks

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Date:   Fri, 29 Sep 2023 11:19:05 -0700
From:   Michael Kelley <mikelley@...rosoft.com>
To:     kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        luto@...nel.org, peterz@...radead.org, thomas.lendacky@....com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        kirill.shutemov@...ux.intel.com, seanjc@...gle.com,
        rick.p.edgecombe@...el.com, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, x86@...nel.org
Cc:     mikelley@...rosoft.com
Subject: [PATCH 1/5] x86/coco: Use slow_virt_to_phys() in page transition hypervisor callbacks

In preparation for temporarily marking pages not present during a
transition between encrypted and decrypted, use slow_virt_to_phys()
in the hypervisor callbacks. As long as the PFN is correct,
slow_virt_to_phys() works even if the leaf PTE is not present.
The existing functions that depends on vmalloc_to_page() all
require that the leaf PTE be marked present, so they don't work.

Update the comments for slow_virt_to_phys() to note this broader usage
and the requirement to work even if the PTE is not marked present.

Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
---
 arch/x86/hyperv/ivm.c        |  9 ++++++++-
 arch/x86/kernel/sev.c        |  8 +++++++-
 arch/x86/mm/pat/set_memory.c | 13 +++++++++----
 3 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
index c1088d3..084fab6 100644
--- a/arch/x86/hyperv/ivm.c
+++ b/arch/x86/hyperv/ivm.c
@@ -524,7 +524,14 @@ static bool hv_vtom_set_host_visibility(unsigned long kbuffer, int pagecount, bo
 		return false;
 
 	for (i = 0, pfn = 0; i < pagecount; i++) {
-		pfn_array[pfn] = virt_to_hvpfn((void *)kbuffer + i * HV_HYP_PAGE_SIZE);
+		/*
+		 * Use slow_virt_to_phys() because the PRESENT bit has been
+		 * temporarily cleared in the PTEs.  slow_virt_to_phys() works
+		 * without the PRESENT bit while virt_to_hvpfn() or similar
+		 * does not.
+		 */
+		pfn_array[pfn] = slow_virt_to_phys((void *)kbuffer +
+					i * HV_HYP_PAGE_SIZE) >> HV_HYP_PAGE_SHIFT;
 		pfn++;
 
 		if (pfn == HV_MAX_MODIFY_GPA_REP_COUNT || i == pagecount - 1) {
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 2787826..f5d6cec 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -784,7 +784,13 @@ static unsigned long __set_pages_state(struct snp_psc_desc *data, unsigned long
 		hdr->end_entry = i;
 
 		if (is_vmalloc_addr((void *)vaddr)) {
-			pfn = vmalloc_to_pfn((void *)vaddr);
+			/*
+			 * Use slow_virt_to_phys() because the PRESENT bit has been
+			 * temporarily cleared in the PTEs.  slow_virt_to_phys() works
+			 * without the PRESENT bit while vmalloc_to_pfn() or similar
+			 * does not.
+			 */
+			pfn = slow_virt_to_phys((void *)vaddr) >> PAGE_SHIFT;
 			use_large_entry = false;
 		} else {
 			pfn = __pa(vaddr) >> PAGE_SHIFT;
diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index bda9f12..8e19796 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -755,10 +755,15 @@ pmd_t *lookup_pmd_address(unsigned long address)
  * areas on 32-bit NUMA systems.  The percpu areas can
  * end up in this kind of memory, for instance.
  *
- * This could be optimized, but it is only intended to be
- * used at initialization time, and keeping it
- * unoptimized should increase the testing coverage for
- * the more obscure platforms.
+ * It is also used in callbacks for CoCo VM page transitions between private
+ * and shared because it works when the PRESENT bit is not set in the leaf
+ * PTE. In such cases, the state of the PTEs, including the PFN, is otherwise
+ * known to be valid, so the returned physical address is correct. The similar
+ * function vmalloc_to_pfn() can't be used because it requires the PRESENT bit.
+ *
+ * This could be optimized, but it is only used in paths that are not perf
+ * sensitive, and keeping it unoptimized should increase the testing coverage
+ * for the more obscure platforms.
  */
 phys_addr_t slow_virt_to_phys(void *__virt_addr)
 {
-- 
1.8.3.1