Re: [PATCH v3 2/2] vhost: Only align sections for vhost-user

[Paolo Bonzini]

On 17/01/20 15:25, Michael S. Tsirkin wrote:
> On Fri, Jan 17, 2020 at 02:58:47PM +0100, Paolo Bonzini wrote:
>> On 17/01/20 14:40, Michael S. Tsirkin wrote:
>>> We don't really need v2 just to add a field. Compatibility is maintained
>>> using feature bits. Adding that is a subject for another patch.
>>> But I'm not sure I understand why does remote need to know about alignment.
>>> This patch seems to handle it locally ...
>>
>> Because the remote vhost here will not be able to use the synic regions.
>>  If it did, it would have the same overlap problem as vhost-kernel.
>>
>> The alignment is needed because, even if you are mapping only [768k,1M)
>> of a 2M hugepage, you need to mmap [0,2M).  You can then discard the
>> rest, but IIUC if you only mmap [768k,1M) then the kernel will fail the
>> mmap.
> 
> So right now remote will query the fd passed to get the alignment.

It should, but will it?  It's not in the spec and I assume QEMU is doing
this alignment work because some server is not doing it.  But indeed we
could use a feature bit to say "don't worry I will be doing the right
thing".

Paolo

> You are basically saying it's not enough in some cases?
>