Re: [Qemu-devel] [PATCH] notdirty_mem_write: implement 8-byte accesses

[Richard Henderson]

On 10/13/2017 11:27 AM, Andrew Baumann wrote:
>> From: Andrew Baumann
>> Sent: Friday, 13 October 2017 11:19
>>
>> Aligned 8-byte memory writes by a 64-bit target on a 64-bit host should
>> always turn into atomic 8-byte writes on the host, however if we missed
>> in the softmmu, and the TLB line was marked as not dirty, then we
>> would end up tearing the 8-byte write into two 4-byte writes in
>> access_with_adjusted_size().
>>
>> Signed-off-by: Andrew Baumann 
>> ---
>> This manifested as a race in lock-free synchronisation with an aarch64
>> Windows guest on an x86-64 host (with multithreaded TCG).
>>
>>  exec.c | 13 +
>>  1 file changed, 13 insertions(+)
> 
> By the way, I noticed that watch_mem_ops are also 4-byte only. I suspect the
> same fix may be needed there?
Yep.


r~

Re: [Qemu-devel] [PATCH] notdirty_mem_write: implement 8-byte accesses

[Richard Henderson]

On 10/13/2017 11:19 AM, Andrew Baumann wrote:
> Aligned 8-byte memory writes by a 64-bit target on a 64-bit host should
> always turn into atomic 8-byte writes on the host, however if we missed
> in the softmmu, and the TLB line was marked as not dirty, then we
> would end up tearing the 8-byte write into two 4-byte writes in
> access_with_adjusted_size().
> 
> Signed-off-by: Andrew Baumann 
> ---
> This manifested as a race in lock-free synchronisation with an aarch64
> Windows guest on an x86-64 host (with multithreaded TCG).

Reviewed-by: Richard Henderson 


r~

Re: [Qemu-devel] [PATCH] notdirty_mem_write: implement 8-byte accesses

[Andrew Baumann via Qemu-devel]

> From: Andrew Baumann
> Sent: Friday, 13 October 2017 11:19
> 
> Aligned 8-byte memory writes by a 64-bit target on a 64-bit host should
> always turn into atomic 8-byte writes on the host, however if we missed
> in the softmmu, and the TLB line was marked as not dirty, then we
> would end up tearing the 8-byte write into two 4-byte writes in
> access_with_adjusted_size().
> 
> Signed-off-by: Andrew Baumann 
> ---
> This manifested as a race in lock-free synchronisation with an aarch64
> Windows guest on an x86-64 host (with multithreaded TCG).
> 
>  exec.c | 13 +
>  1 file changed, 13 insertions(+)

By the way, I noticed that watch_mem_ops are also 4-byte only. I suspect the 
same fix may be needed there?

Andrew

[Qemu-devel] [PATCH] notdirty_mem_write: implement 8-byte accesses

[Andrew Baumann via Qemu-devel]

Aligned 8-byte memory writes by a 64-bit target on a 64-bit host should
always turn into atomic 8-byte writes on the host, however if we missed
in the softmmu, and the TLB line was marked as not dirty, then we
would end up tearing the 8-byte write into two 4-byte writes in
access_with_adjusted_size().

Signed-off-by: Andrew Baumann 
---
This manifested as a race in lock-free synchronisation with an aarch64
Windows guest on an x86-64 host (with multithreaded TCG).

 exec.c | 13 +
 1 file changed, 13 insertions(+)

diff --git a/exec.c b/exec.c
index 6378714a2b..7c591a9b75 100644
--- a/exec.c
+++ b/exec.c
@@ -2348,6 +2348,9 @@ static void notdirty_mem_write(void *opaque, hwaddr 
ram_addr,
 case 4:
 stl_p(qemu_map_ram_ptr(NULL, ram_addr), val);
 break;
+case 8:
+stq_p(qemu_map_ram_ptr(NULL, ram_addr), val);
+break;
 default:
 abort();
 }
@@ -2378,6 +2381,16 @@ static const MemoryRegionOps notdirty_mem_ops = {
 .write = notdirty_mem_write,
 .valid.accepts = notdirty_mem_accepts,
 .endianness = DEVICE_NATIVE_ENDIAN,
+.valid = {
+.min_access_size = 1,
+.max_access_size = 8,
+.unaligned = false,
+},
+.impl = {
+.min_access_size = 1,
+.max_access_size = 8,
+.unaligned = false,
+},
 };
 
 /* Generate a debug exception if a watchpoint has been hit.  */
-- 
2.14.2