Re: [Qemu-devel] [PATCH] kvm: First step to push iothread lock out of inner run loop

[Jan Kiszka <jan.kiszka@web.de>]

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On 2012-06-24 16:56, Avi Kivity wrote:
> On 06/24/2012 05:51 PM, Jan Kiszka wrote:
>> On 2012-06-24 16:46, Avi Kivity wrote:
>>> On 06/24/2012 05:40 PM, Jan Kiszka wrote:
>>>> On 2012-06-24 16:35, Avi Kivity wrote:
>>>>> On 06/24/2012 05:08 PM, Jan Kiszka wrote:
>>>>>> As a first step, I will post a series later that gets rid of
>>>>>> kvm_flush_coalesced_mmio_buffer in the common vmexit path.
>>>>>
>>>>> If you defer this, I can think of two places that need to flush:
>>>>> - anything that accesses those memory areas (such as DMA to the
>>>>> framebuffer, or updating the display)
>>>>
>>>> - anything that accesses related areas (in case of VGA: PIO accesses to
>>>> the control ports). I'm providing memory_region_set_flush_coalesced that
>>>> allows to flush on non-coalesced region accesses as well. Some PIO
>>>> accesses unfortunately still need open-coded
>>>> qemu_flush_coalesced_mmio_buffer as they do not use memory regions yet.
>>>
>>> Framebuffer access will bypass the MemoryRegionOps callbacks, did you
>>> intend to hook those?
>>
>> Are there really cases where the framebuffer is accessible both via MMIO
>> and RAM-like mappings at the same time? If so, the current flushing on
>> vmexit would help either as the direct mappings would not trigger exits.
>> Or what do you mean?
> 
> I meant accesses by the display code to put the framebuffer in front of
> the user's eyes.  Those just access the memory directly.  We could wrap
> them with memory_region_get/put_ram_ptr() (as Xen requires anyway) and
> have those issue the flush.

That's a non issue, we already have to flush on display updates. See
e.g. vga_update_display.

Jan


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