Re: [Qemu-devel] [PATCH 3/4] memory: introduce memory_global_after_dirty_log_sync

[Richard Henderson]

On 02/09/2018 02:45 AM, Paolo Bonzini wrote:
> There is a race between TCG and accesses to the dirty log:
> 
>       vCPU thread                  reader thread
>       -----------------------      -----------------------
>       TLB check -> slow path
>         notdirty_mem_write
>           write to RAM
>           set dirty flag
>                                    clear dirty flag
>       TLB check -> fast path
>                                    read memory
>         write to RAM
> 
> Fortunately, in order to fix it, no change is required to the
> vCPU thread.  However, the reader thread must delay the read after
> the vCPU thread has finished the write.  This can be approximated
> conservatively by run_on_cpu, which waits for the end of the current
> translation block.
> 
> A similar technique is used by KVM, which has to do a synchronous TLB
> flush after doing a test-and-clear of the dirty-page flags.
> 
> Reported-by: Dr. David Alan Gilbert <address@hidden>
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  exec.c                | 31 +++++++++++++++++++++++++++++++
>  include/exec/memory.h | 12 ++++++++++++
>  memory.c              | 10 +++++++++-
>  migration/ram.c       |  1 +
>  4 files changed, 53 insertions(+), 1 deletion(-)

Reviewed-by: Richard Henderson <address@hidden>


r~