target/arm:Set lg_page_size to 0 if either S1 or S2 asks for it

In get_phys_addr_twostage() we set the lg_page_size of the result to
the maximum of the stage 1 and stage 2 page sizes.  This works for
the case where we do want to create a TLB entry, because we know the
common TLB code only creates entries of the TARGET_PAGE_SIZE and
asking for a size larger than that only means that invalidations
invalidate the whole larger area.  However, if lg_page_size is
smaller than TARGET_PAGE_SIZE this effectively means "don't create a
TLB entry"; in this case if either S1 or S2 said "this covers less
than a page and can't go in a TLB" then the final result also should
be marked that way.  Set the resulting page size to 0 if either
stage asked for a less-than-a-page entry, and expand the comment
to explain what's going on.

This has no effect for VMSA because currently the VMSA lookup always
returns results that cover at least TARGET_PAGE_SIZE; however when we
add v8R support it will reuse this code path, and for v8R the S1 and
S2 results can be smaller than TARGET_PAGE_SIZE.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20221212142708.610090-1-peter.maydell@linaro.org

1 files changed, 13 insertions, 3 deletions

diff --git a/target/arm/ptw.c b/target/arm/ptw.c
index f812734bfb..2e7826dc29 100644
--- a/target/arm/ptw.c
+++ b/target/arm/ptw.c
@@ -2655,10 +2655,20 @@ static bool get_phys_addr_twostage(CPUARMState *env, S1Translate *ptw,
     }
 
     /*
-     * Use the maximum of the S1 & S2 page size, so that invalidation
-     * of pages > TARGET_PAGE_SIZE works correctly.
+     * If either S1 or S2 returned a result smaller than TARGET_PAGE_SIZE,
+     * this means "don't put this in the TLB"; in this case, return a
+     * result with lg_page_size == 0 to achieve that. Otherwise,
+     * use the maximum of the S1 & S2 page size, so that invalidation
+     * of pages > TARGET_PAGE_SIZE works correctly. (This works even though
+     * we know the combined result permissions etc only cover the minimum
+     * of the S1 and S2 page size, because we know that the common TLB code
+     * never actually creates TLB entries bigger than TARGET_PAGE_SIZE,
+     * and passing a larger page size value only affects invalidations.)
      */
-    if (result->f.lg_page_size < s1_lgpgsz) {
+    if (result->f.lg_page_size < TARGET_PAGE_BITS ||
+        s1_lgpgsz < TARGET_PAGE_BITS) {
+        result->f.lg_page_size = 0;
+    } else if (result->f.lg_page_size < s1_lgpgsz) {
         result->f.lg_page_size = s1_lgpgsz;
     }