Re: [PATCH] Avoid fragment virtio-blk transfers by copying

[Anthony Liguori <aliguori@us.ibm.com>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> A major source of performance loss for virtio-blk has been the fact 
>> that we
>> split transfers into multiple requests.  This is particularly harmful 
>> if you
>> have striped storage beneath your virtual machine.
>>
>> This patch copies the request data into a single contiguous buffer to 
>> ensure
>> that we don't split requests.  This improves performance from about 
>> 80 MB/sec
>> to about 155 MB/sec with my fibre channel link.  185 MB/sec is what 
>> we get on
>> native so this gets us pretty darn close.
>>
>>   
>
> If the guest issues a request for a terabyte of memory, the host will 
> try to allocate it and drop to swap/oom.

An unprivileged user should not be able to OOM the kernel by simply 
doing memory allocations.  Likewise, while it will start swapping, that 
should primarily effect the application allocating memory (although yes, 
it's consuming IO bandwidth to do the swapping).

As long as we properly handle memory allocation failures, it's my 
contention that allowing a guest to allocate unbound amounts of virtual 
memory is safe.

>   So we need to either fragment beyond some size, or to avoid copying 
> and thus the need for allocation.

As Marcelo mentioned, there are very practical limitations on how much 
memory can be in-flight on any given queue.  A malicious guest could 
construct a nasty queue that basically pointed to all of the guest 
physical memory for each entry in the queue but that's still a bound size.

Regards,

Anthony Liguori