Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT)

[Avi Kivity <avi@qumranet.com>]

Jamie Lokier wrote:
> Avi Kivity wrote:
>   
>> Here's a summary of the use cases I saw so far:
>>
>> - casual use, no critical data: write back cache
>>
>> - backing file shared among many guests: read-only, cached
>>
>> - desktop system, but don't lose my data: O_SYNC
>> (significant resources on the host)
>>
>> - dedicated virtualization engine: O_DIRECT
>> (most host resources assigned to guests)
>>     
>
> Sounds alright, but on _my_ desktop system (a laptop), I would use O_DIRECT.
>
> There isn't enough RAM in my system to be happy duplicating data in
> guests and hosts at the same time.  VMs are quite demanding on RAM.
>
>   

Sure, if you're low on resources, and aren't rebooting often, that's the 
right thing to do.

> However, if you find a way to map host cached pages into the guest
> without copying - so sharing the RAM - that would be excellent.  It
> can be done in principle, by remapping pages to satisfy IDE/SCSI DMA
> requests.  I don't know if it would be fast enough.  Perhaps it would
> work better in KVM than QEMU.
>   

Sounds like a memory management nightmare.  With mmu notifiers (or plain 
qemu), though, it can be done.  Have the backing file also contain an 
area for guest RAM.  Use a nonlinear mapping to map this area as guest 
memory.  If the guest issues a properly-aligned read, call 
remap_file_pages() for that page, and write-protect it.  When you get a 
protection violation (as the guest writes to that memory), copy it to 
the RAM area and remap it again.

I don't think remap_file_pages() supports different protections in a 
single VMA; that could kill the whole idea.

-- 
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