Re: [PATCH v4 14/14] KVM: ARM: Handle I/O aborts

Will Deacon
2012-11-19T15:09:41Z

On Sat, Nov 10, 2012 at 03:43:49PM +0000, Christoffer Dall wrote:
> When the guest accesses I/O memory this will create data abort
> exceptions and they are handled by decoding the HSR information
> (physical address, read/write, length, register) and forwarding reads
> and writes to QEMU which performs the device emulation.
>
> Certain classes of load/store operations do not support the syndrome
> information provided in the HSR and we therefore must be able to fetch
> the offending instruction from guest memory and decode it manually.
>
> We only support instruction decoding for valid reasonable MMIO operations
> where trapping them do not provide sufficient information in the HSR (no
> 16-bit Thumb instructions provide register writeback that we care about).
>
> The following instruction types are NOT supported for MMIO operations
> despite the HSR not containing decode info:
> - any Load/Store multiple
> - any load/store exclusive
> - any load/store dual
> - anything with the PC as the dest register
>
> This requires changing the general flow somewhat since new calls to run
> the VCPU must check if there's a pending MMIO load and perform the write
> after userspace has made the data available.
>
> Rusty Russell fixed a horrible race pointed out by Ben Herrenschmidt:
> (1) Guest complicated mmio instruction traps.
> (2) The hardware doesn't tell us enough, so we need to read the actual
> instruction which was being exectuted.
> (3) KVM maps the instruction virtual address to a physical address.
> (4) The guest (SMP) swaps out that page, and fills it with something else.
> (5) We read the physical address, but now that's the wrong thing.
>
> Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
> Signed-off-by: Rusty Russell <rusty.russell@linaro.org>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>

This is looking like the right sort of thing now, but I would like to
see an Acked-by from Dave [CC'd] for this patch.

I'll try and hit the vGIC code this week...

Thanks,

Will