Re: [Xen-devel] [PATCH RFC v2 6/9] xen/arm: Add handling write fault for dirty-page tracing

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-07-03 at 18:15 +0900, Jaeyong Yoo wrote:
> From: Elena Pyatunina <e.pyatunina@xxxxxxxxxxx>
> 
> Add handling write fault in do_trap_data_abort_guest for dirty-page
> tracing. Mark dirty function makes the bitmap of dirty pages.

You've gone with the model of allocating a bitmap when logdirty is
enabled and updating it directly from the fault handler? The bit map is
based around a three level structure mirroring the p2m tables?

I had been considering the option of reusing one of the bits available
for software use in the p2m entries to track the dirty status and
walking the p2m when the toolstack asked for the current bitmap instead.

I think this would have less memory overhead and also would allow us to
be lock free on the fault handling path (by using suitable atomic
exchanges on the p2m entries), as well as avoiding memory allocations on
that path.

I think we'd want to establish a linear map of the current guest p2m for
these purposes so we could also avoid all the map_domain_page stuff in
the fault path. I think there is enough virtual address space left for
that even on 32-bit, it's less critical on 64-bit anyway since we are
about to have a direct map of RAM available to us.

> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> index 398d209..9927712 100644
> --- a/xen/arch/arm/traps.c
> +++ b/xen/arch/arm/traps.c
> @@ -1011,6 +1011,13 @@ static void do_trap_data_abort_guest(struct 
> cpu_user_regs *regs,
>      if ( rc == -EFAULT )
>          goto bad_data_abort;
>  
> +    /* handle permission fault on write */
> +    if ((dabt.dfsc & 0x3f) == (FSC_FLT_PERM + 3) && dabt.write)

Can we get some #defines for these magic constants please.

> +    {
> +        if (handle_page_fault(current->domain, info.gpa) == 0)
> +            return;
> +    }
> +
>      if (handle_mmio(&info))

Since you handle page fault first, won't this make MMIO trapping regions
writable?

Ian.


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