Re: [Qemu-devel] [PATCH 2/2] target/arm: lie more convincingly about memory attributes in PAR_EL1

[Peter Maydell]

On 28 February 2017 at 21:58, Andrew Baumann
 wrote:
> On a successful long-descriptor translation, PAR_EL1 bits 56:63 are
> expected to report the memory attributes of the page. However, the
> page table walker (get_phys_addr()) does not currently retrieve these
> attributes. Rather than leaving these bits clear (which implies
> uncacheable device memory), this change sets them to 0xff, which
> corresponds to write-back cached normal memory.
>
> Signed-off-by: Andrew Baumann 
> ---
> In my (biased!) opinion, this is a better lie to tell for an emulated
> environment. It also happens to un-break the Windows boot process, and
> enable an NT kernel optimisation (DC ZVA for page zeroing).

It's not clear to me that misreporting Device memory as
Normal is any better than misreporting Normal memory as
Device -- at least, it seems like it might break currently
working guests.

I think it would be safer to implement the PAR as "honour the
attribute index bits in page table descriptors and return
the corresponding bits from the MAIR registers".

thanks
-- PMM

[Qemu-devel] [PATCH 2/2] target/arm: lie more convincingly about memory attributes in PAR_EL1

[Andrew Baumann]

On a successful long-descriptor translation, PAR_EL1 bits 56:63 are
expected to report the memory attributes of the page. However, the
page table walker (get_phys_addr()) does not currently retrieve these
attributes. Rather than leaving these bits clear (which implies
uncacheable device memory), this change sets them to 0xff, which
corresponds to write-back cached normal memory.

Signed-off-by: Andrew Baumann 
---
In my (biased!) opinion, this is a better lie to tell for an emulated
environment. It also happens to un-break the Windows boot process, and
enable an NT kernel optimisation (DC ZVA for page zeroing).

 target/arm/helper.c | 8 +++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index 760092a..b858d6d 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2159,7 +2159,13 @@ static uint64_t do_ats_write(CPUARMState *env, uint64_t 
value,
 if (!attrs.secure) {
 par64 |= (1 << 9); /* NS */
 }
-/* We don't set the ATTR or SH fields in the PAR. */
+/* ATTR bits are all set, which implies:
+ *   normal memory
+ *   outer write-back non-transient, read/write-allocate
+ *   inner write-back non-transient, read/write-allocate
+ */
+par64 |= ((uint64_t)0xff << 56); /* ATTR */
+/* We don't set the SH field in the PAR. */
 } else {
 par64 |= 1; /* F */
 par64 |= (fsr & 0x3f) << 1; /* FS */
-- 
2.8.3