Re: [PATCH] KVM: x86: fix vcpu->mmio_fragments overflow

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-10-22 14:53, Gleb Natapov wrote:
> On Mon, Oct 22, 2012 at 02:45:37PM +0200, Jan Kiszka wrote:
>> On 2012-10-22 14:18, Avi Kivity wrote:
>>> On 10/22/2012 01:45 PM, Jan Kiszka wrote:
>>>
>>>>> Indeed. git pull, recheck and call for kvm_flush_coalesced_mmio_buffer()
>>>>> is gone. So this will break new userspace, not old. By global you mean
>>>>> shared between devices (or memory regions)?
>>>>
>>>> Yes. We only have a single ring per VM, so we cannot flush multi-second
>>>> VGA access separately from other devices. In theory solvable by
>>>> introducing per-region rings that can be driven separately.
>>>
>>> But in practice unneeded.  Real time VMs can disable coalescing and not
>>> use planar VGA modes.
>>
>> A) At least right now, we do not differentiate between the VGA modes and
>> if flushing is needed. So that device is generally taboo for RT cores of
>> the VM.
>> B) We need to disable coalescing in E1000 as well - if we want to use
>> that model.
>> C) Gleb seems to propose using coalescing far beyond those two use cases.
>>
> Since the userspace change is needed the idea is dead, but if we could
> implement it I do not see how it can hurt the latency if it would be the
> only mechanism to use coalesced mmio buffer. Checking that the ring buffer
> is empty is cheap and if it is not empty it means that kernel just saved
> you a lot of 8 bytes exists so even after iterating over all the entries there
> you still saved a lot of time.

When taking an exit for A, I'm not interesting in flushing stuff for B
unless I have a dependency. Thus, buffers would have to be per device
before extending their use.

Jan

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