i386/hvf: Fixes dirty memory tracking by page granularity RX->RWX change

When using x86 macOS Hypervisor.framework as accelerator, detection of
dirty memory regions is implemented by marking logged memory region
slots as read-only in the EPT, then setting the dirty flag when a
guest write causes a fault. The area marked dirty should then be marked
writable in order for subsequent writes to succeed without a VM exit.

However, dirty bits are tracked on a per-page basis, whereas the fault
handler was marking the whole logged memory region as writable. This
change fixes the fault handler so only the protection of the single
faulting page is marked as dirty.

(Note: the dirty page tracking appeared to work despite this error
because HVF’s hv_vcpu_run() function generated unnecessary EPT fault
exits, which ended up causing the dirty marking handler to run even
when the memory region had been marked RW. When using
hv_vcpu_run_until(), a change planned for a subsequent commit, these
spurious exits no longer occur, so dirty memory tracking malfunctions.)

Additionally, the dirty page is set to permit code execution, the same
as all other guest memory; changing memory protection from RX to RW not
RWX appears to have been an oversight.

Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Roman Bolshakov <roman@roolebo.dev>
Tested-by: Roman Bolshakov <roman@roolebo.dev>
Message-ID: <20240605112556.43193-5-phil@philjordan.eu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/target/i386/hvf/hvf.c b/target/i386/hvf/hvf.c
index e6e9162..268c573 100644
--- a/target/i386/hvf/hvf.c
+++ b/target/i386/hvf/hvf.c
@@ -135,9 +135,10 @@ static bool ept_emulation_fault(hvf_slot *slot, uint64_t gpa, uint64_t ept_qual)
 
     if (write && slot) {
         if (slot->flags & HVF_SLOT_LOG) {
+            uint64_t dirty_page_start = gpa & ~(TARGET_PAGE_SIZE - 1u);
             memory_region_set_dirty(slot->region, gpa - slot->start, 1);
-            hv_vm_protect((hv_gpaddr_t)slot->start, (size_t)slot->size,
-                          HV_MEMORY_READ | HV_MEMORY_WRITE);
+            hv_vm_protect(dirty_page_start, TARGET_PAGE_SIZE,
+                          HV_MEMORY_READ | HV_MEMORY_WRITE | HV_MEMORY_EXEC);
         }
     }