Re: [Xen-devel] Upstream Dom0 DRM problems regarding swiotlb

[Juergen Gross <jgross@xxxxxxxx>]

On 13/02/2019 19:38, Michael Labriola wrote:
> On Wed, Feb 13, 2019 at 1:16 PM Michael Labriola
> <michael.d.labriola@xxxxxxxxx> wrote:
>>
>> On Wed, Feb 13, 2019 at 11:57 AM Konrad Rzeszutek Wilk
>> <konrad.wilk@xxxxxxxxxx> wrote:
>>>
>>> On Wed, Feb 13, 2019 at 09:09:32AM -0700, Jan Beulich wrote:
>>>>>>> On 13.02.19 at 17:00, <michael.d.labriola@xxxxxxxxx> wrote:
>>>>> On Wed, Feb 13, 2019 at 9:28 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>>>>> On 13.02.19 at 15:10, <michael.d.labriola@xxxxxxxxx> wrote:
>>>>>>> Ah, so this isn't necessarily Xen-specific but rather any paravirtual
>>>>>>> guest?  That hadn't crossed my mind.  Is there an easy way to find out
>>>>>>> if we're a pv guest in the need_swiotlb conditionals?
>>>>>>
>>>>>> There's xen_pv_domain(), but I think xen_swiotlb would be more to
>>>>>> the point if the check is already to be Xen-specific. There's no generic
>>>>>> "is PV" predicate that I'm aware of.
>>>>>
>>>>> Well, that makes doing conditional code right more difficult.  I
>>>>> assume since there isn't a generic predicate, and PV isn't new, that
>>>>> it's absence is by design?  To reign in the temptation to sprinkle
>>>>> conditional code all over the kernel?  ;-)
>>>>
>>>> Well, with lguest gone, Xen is the only PV environment the kernel
>>>> can run in, afaik at least. I guess to decide between the suggested
>>>> options or the need for some abstracting macro (or yet something
>>>> else), you'll really need to ask the driver maintainers. Or simply
>>>> send a patch their way implementing one of them, and see what
>>>> their reaction is.
>>>
>>> Could you try this out and see if it works and I will send it out:
>>>
> *snip*
>>
>> Yes, that works for me.  However, I feel like the conditional should
>> be in drm_get_max_iomem() instead of directly after it everywhere it's
>> used...  and is just checking xen_pv_domain() enough?  Jan made it
>> sound like there were possibly other PV cases that would also still
>> need swiotlb.
> 
> How about this?  It strcmp's pv_info to see if we're bare metal, does
> the comparison in a single place, moves the bit shifting comparison
> into the function (simplifying the drm driver code), and renames the
> function to more aptly describe what's going on.

...

> diff --git a/drivers/gpu/drm/drm_memory.c b/drivers/gpu/drm/drm_memory.c
> index d69e4fc1ee77..f22f6a0d20b3 100644
> --- a/drivers/gpu/drm/drm_memory.c
> +++ b/drivers/gpu/drm/drm_memory.c
> @@ -35,6 +35,7 @@
> 
>  #include <linux/highmem.h>
>  #include <linux/export.h>
> +#include <xen/xen.h>
>  #include <drm/drmP.h>
>  #include "drm_legacy.h"
> 
> @@ -150,15 +151,24 @@ void drm_legacy_ioremapfree(struct drm_local_map
> *map, struct drm_device *dev)
>  }
>  EXPORT_SYMBOL(drm_legacy_ioremapfree);
> 
> -u64 drm_get_max_iomem(void)
> +bool drm_need_swiotlb_for_dma(int dma_bits)
>  {
>      struct resource *tmp;
>      resource_size_t max_iomem = 0;
> 
> +#ifdef CONFIG_PARAVIRT
> +    /*
> +     * Paravirtual hosts require swiotlb regardless of requested dma
> +     * transfer size.
> +     */
> +    if (strcmp(pv_info.name, "bare hardware") != 0)
> +        return true;
> +#endif
> +

No, this is really not acceptable.

Apart from Andrew's comments on using the DMA API (which I really
support) relying on the pv_info.name string is a very bad interface.
You'd need to add something like a "need_swiotlb" boolean to the
pv_info struct and set it for Xen PV and maybe others.


Juergen

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