Re: [Qemu-devel] [PATCH] exec: Fix MAP_RAM for cached access

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/06/2018 21:05, Eric Auger wrote:
> When an IOMMUMemoryRegion is in front of a virtio device,
> address_space_cache_init does not set cache->ptr as the memory
> region is not RAM. However when the device performs an access,
> we end up in glue() which performs the translation and then uses
> MAP_RAM. This latter uses the unset ptr and returns a wrong value
> which leads to a SIGSEV in address_space_lduw_internal_cached_slow,
> for instance. Let's test whether the cache->ptr is set, and in
> the negative use the old macro definition. This fixes the
> use cases featuring vIOMMU (Intel and ARM SMMU) which lead to
> a SIGSEV.
> 
> Fixes: 48564041a73a (exec: reintroduce MemoryRegion caching)
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> 
> ---
> 
> I am not sure whether it doesn't break any targeted optimization
> but at least it removes the SIGSEV.

Actually cache->ptr is always NULL here, since this is the slow path
(there is even an assertion in address_space_translate_cached); so
MAP_RAM can be even simpler and, apart from the bugfix, I think we
should remove all of IS_DIRECT, MAP_RAM and INVALIDATE as a follow-up.
They were needed in the original implementation of MemoryRegionCache,
which only worked with RAM regions but not anymore now that the RAM case
is open-coded in include/exec/memory_ldst_cached.inc.h.

Thanks,

Paolo

> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> ---
>  exec.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/exec.c b/exec.c
> index f6645ed..46fbd25 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -3800,7 +3800,9 @@ address_space_write_cached_slow(MemoryRegionCache *cache, hwaddr addr,
>  #define SUFFIX                   _cached_slow
>  #define TRANSLATE(...)           address_space_translate_cached(cache, __VA_ARGS__)
>  #define IS_DIRECT(mr, is_write)  memory_access_is_direct(mr, is_write)
> -#define MAP_RAM(mr, ofs)         (cache->ptr + (ofs - cache->xlat))
> +#define MAP_RAM(mr, ofs)         (cache->ptr ? \
> +                                 (cache->ptr + (ofs - cache->xlat)) :  \
> +                                 qemu_map_ram_ptr((mr)->ram_block, ofs))
>  #define INVALIDATE(mr, ofs, len) invalidate_and_set_dirty(mr, ofs, len)
>  #define RCU_READ_LOCK()          ((void)0)
>  #define RCU_READ_UNLOCK()        ((void)0)
>