Re: [Xen-devel] Assigning contiguous memory to a driver domain

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 15.09.10 at 11:39, Rafal Wojtczuk <rafal@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Sep 15, 2010 at 11:08:42AM +0200, Rafal Wojtczuk wrote:
>> On Tue, Sep 14, 2010 at 10:36:56AM +0100, Jan Beulich wrote:
>> > >>> On 14.09.10 at 11:24, Rafal Wojtczuk <rafal@xxxxxxxxxxxxxxxxxxxxxx> 
>> > >>> wrote:
>> > > Hello,
>> > > 
>> > > Could someone guide me in the right direction with the topic of assigning
>> > > contiguous memory to a domain.
>> > > 
>> > > I have an issue with a PV domain that is assigned a PCI device. 
>> > > Sometimes, 
> 
>> > > the 
>> > > driver fails to load
>> > > Sep 13 10:36:43 localhost kernel: [  103.651858] iwlagn 0000:00:01.0:
>> > > firmware: requesting iwlwifi-4965-2.ucode
>> > > Sep 13 10:36:43 localhost kernel: [  103.669105] iwlagn 0000:00:01.0: 
> loaded
>> > > firmware version 228.61.2.24
>> > > Sep 13 10:36:43 localhost kernel: [  103.669263] iwlagn 0000:00:01.0: 
> failed
>> > > to allocate pci memory
>> > > 
>> > > The reason seems to be that the domain does not have enough contiguous
>> > > memory, in mfn terms.
>> > 
>> > No, how (dis)contiguous the memory of a domain is doesn't matter
>> > here. What matters is whether the domain can *make* the requested
>> > memory contiguous, and that depends on how much contiguous
>> > memory Xen has at the point of the allocation.
>> 
>> Ah, so you are saying that regardless of whether a domain has some
>> contiguous memory, the driver will call xen_create_contiguous_region when 
> allocating
>> memory (via dma_alloc_coherent ?).
>> 
>> Slightly out-of-the-list-scope: is there a convention when a driver should
>> allocate DMA-able memory ? Is it safe to assume that as soon as the driver
>> has loaded, it will no longer need to call xen_create_contiguous_region
>> anymore and we can use up all free Xen memory ? 
> 
> Hmm, at least in case of tg3 driver, if xen free memory = 0, after I have
> done "ifconfig eth0 down" in the driver domain, the subsequent "ifconfig 
> eth0 up" 
> failed with "SIOCSIFFLAGS: Cannot allocate memory".

Sure - why would the driver waste resources when the device may
not be used.

> So, it looks like in order to make a PV driver domain work, there must be 
> some Xen free memory 
> all the time ?

Potentially yes, but this really depends on how the respective
driver is written.

> Moreover, this free Xen memory must be contiguous to some
> extent; is there any way to assure this ?

No.

Jan


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