Re: [PATCH v5 13/14] KVM: ARM: Handle I/O aborts

Gleb Natapov
2013-01-15T13:35:40Z

On Tue, Jan 15, 2013 at 01:29:40PM +0000, Marc Zyngier wrote:
> On 15/01/13 13:18, Gleb Natapov wrote:
> > On Tue, Jan 08, 2013 at 01:40:05PM -0500, 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.
> > How can this happen?! The guest cannot reuse physical page before it
> > flushes it from all vcpus tlb cache. For that it needs to send
> > synchronous IPI to all vcpus and IPI will not be processed by a vcpu
> > while it does emulation.
>
> I don't know how this works on x86, but a KVM/ARM guest can definitely
> handle an IPI.
>
How can a vcpu handle an IPI while it is not in a guest mode?

> Furthermore, TLB invalidation doesn't require an IPI on ARMv7 (unless
> we're doing some set/way operation which is handled separately).
>
What prevents a page to be swapped out while code is fetched from it?

--
Gleb.