[PATCH v2] target-i386: Walk NPT in guest real mode

[Alexander Graf <graf@amazon.com>]

When translating virtual to physical address with a guest CPU that
supports nested paging (NPT), we need to perform every page table walk
access indirectly through the NPT, which we correctly do.

However, we treat real mode (no page table walk) special: In that case,
we currently just skip any walks and translate VA -> PA. With NPT
enabled, we also need to then perform NPT walk to do GVA -> GPA -> HPA
which we fail to do so far.

The net result of that is that TCG VMs with NPT enabled that execute
real mode code (like SeaBIOS) end up with GPA==HPA mappings which means
the guest accesses host code and data. This typically shows as failure
to boot guests.

This patch changes the page walk logic for NPT enabled guests so that we
always perform a GVA -> GPA translation and then skip any logic that
requires an actual PTE.

That way, all remaining logic to walk the NPT stays and we successfully
walk the NPT in real mode.

Fixes: fe441054bb3f0 ("target-i386: Add NPT support")

Signed-off-by: Alexander Graf <graf@amazon.com>
Reported-by: Eduard Vlad <evlad@amazon.de>

---

v1 -> v2:

  - Remove hack where we fake a PTE and instead just set the
    corresponding resolved variables and jump straight to the
    stage2 code.
---
 target/i386/tcg/sysemu/excp_helper.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/target/i386/tcg/sysemu/excp_helper.c b/target/i386/tcg/sysemu/excp_helper.c
index 8fb05b1f53..4622d45643 100644
--- a/target/i386/tcg/sysemu/excp_helper.c
+++ b/target/i386/tcg/sysemu/excp_helper.c
@@ -298,7 +298,7 @@ static bool mmu_translate(CPUX86State *env, const TranslateParams *in,
         /* combine pde and pte nx, user and rw protections */
         ptep &= pte ^ PG_NX_MASK;
         page_size = 4096;
-    } else {
+    } else if (pg_mode) {
         /*
          * Page table level 2
          */
@@ -343,6 +343,12 @@ static bool mmu_translate(CPUX86State *env, const TranslateParams *in,
         ptep &= pte | PG_NX_MASK;
         page_size = 4096;
         rsvd_mask = 0;
+    } else {
+        /* No paging (real mode), let's assemble a fake 1:1 1GiB PTE */
+        page_size = 0x40000000;
+        paddr = in->addr;
+        prot = PAGE_READ | PAGE_WRITE | PAGE_EXEC;
+        goto stage2;
     }
 
 do_check_protect:
@@ -420,6 +426,7 @@ do_check_protect_pse36:
 
     /* merge offset within page */
     paddr = (pte & PG_ADDRESS_MASK & ~(page_size - 1)) | (addr & (page_size - 1));
+stage2:
 
     /*
      * Note that NPT is walked (for both paging structures and final guest
@@ -562,7 +569,7 @@ static bool get_physical_address(CPUX86State *env, vaddr addr,
             addr = (uint32_t)addr;
         }
 
-        if (likely(env->cr[0] & CR0_PG_MASK)) {
+        if (likely(env->cr[0] & CR0_PG_MASK || use_stage2)) {
             in.cr3 = env->cr[3];
             in.mmu_idx = mmu_idx;
             in.ptw_idx = use_stage2 ? MMU_NESTED_IDX : MMU_PHYS_IDX;
-- 
2.40.1




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