Re: [PATCH V2 02/14] virtio-pci: switch to use devres for modern devices

[Jason Wang]

On 2020/11/26 下午9:57, Michael S. Tsirkin wrote:
> On Thu, Nov 26, 2020 at 05:25:52PM +0800, Jason Wang wrote:
> > This patch tries to convert the modern device to use devres to manage
> > its resources (iomaps). Before this patch the IO address is mapped
> > individually according to the capability. After this patch, we simply
> > map the whole BAR.
> I think the point of mapping capability was e.g. for devices with
> huge BARs. We don't want to waste virtual memory for e.g. 32 bit guests.
>
> And in particular the spec says:
>
> 	The drivers SHOULD only map part of configuration structure large enough for device operation. The drivers
> 	MUST handle an unexpectedly large length, but MAY check that length is large enough for device operation.


Good point, so I will stick to devres but not use the shortcut like 
whole BAR mapping.


> I also wonder how would this interact with cases where device memory is
> mapped for different reasons, such as for MSI table access, into userspace
> as it has resources such as virtio mem, etc.


I think it depends on the driver, e.g for virtio-pci and vDPA, the upper 
layer driver (virtio bus or vDPA bus) know nothing about transport 
specific thing. It should be ok.


> E.g. don't e.g. intel CPUs disallow mapping the same address twice
> with different attributes?


Do you mean it doesn't allow one VA is mapped as UC but the other is 
not? I don't know. But anyhow my understanding is that 
virtio-pci/vp_vdpa tries to hide the details so we can not have two 
mappings here.

Thanks


>