Re: [PATCH v3 2/5] xen/common: Guard iommu symbols with CONFIG_HAS_PASSTHROUGH

[Julien Grall <julien@xen.org>]

On 18/05/2021 07:27, Jan Beulich wrote:
> On 18.05.2021 06:11, Connor Davis wrote:
>>
>> On 5/17/21 9:42 AM, Julien Grall wrote:
>>> Hi Jan,
>>>
>>> On 17/05/2021 12:16, Jan Beulich wrote:
>>>> On 14.05.2021 20:53, Connor Davis wrote:
>>>>> --- a/xen/common/memory.c
>>>>> +++ b/xen/common/memory.c
>>>>> @@ -294,7 +294,9 @@ int guest_remove_page(struct domain *d, unsigned
>>>>> long gmfn)
>>>>>        p2m_type_t p2mt;
>>>>>    #endif
>>>>>        mfn_t mfn;
>>>>> +#ifdef CONFIG_HAS_PASSTHROUGH
>>>>>        bool *dont_flush_p, dont_flush;
>>>>> +#endif
>>>>>        int rc;
>>>>>      #ifdef CONFIG_X86
>>>>> @@ -385,13 +387,17 @@ int guest_remove_page(struct domain *d,
>>>>> unsigned long gmfn)
>>>>>         * Since we're likely to free the page below, we need to suspend
>>>>>         * xenmem_add_to_physmap()'s suppressing of IOMMU TLB flushes.
>>>>>         */
>>>>> +#ifdef CONFIG_HAS_PASSTHROUGH
>>>>>        dont_flush_p = &this_cpu(iommu_dont_flush_iotlb);
>>>>>        dont_flush = *dont_flush_p;
>>>>>        *dont_flush_p = false;
>>>>> +#endif
>>>>>          rc = guest_physmap_remove_page(d, _gfn(gmfn), mfn, 0);
>>>>>    +#ifdef CONFIG_HAS_PASSTHROUGH
>>>>>        *dont_flush_p = dont_flush;
>>>>> +#endif
>>>>>          /*
>>>>>         * With the lack of an IOMMU on some platforms, domains with
>>>>> DMA-capable
>>>>> @@ -839,11 +845,13 @@ int xenmem_add_to_physmap(struct domain *d,
>>>>> struct xen_add_to_physmap *xatp,
>>>>>        xatp->gpfn += start;
>>>>>        xatp->size -= start;
>>>>>    +#ifdef CONFIG_HAS_PASSTHROUGH
>>>>>        if ( is_iommu_enabled(d) )
>>>>>        {
>>>>>           this_cpu(iommu_dont_flush_iotlb) = 1;
>>>>>           extra.ppage = &pages[0];
>>>>>        }
>>>>> +#endif
>>>>>          while ( xatp->size > done )
>>>>>        {
>>>>> @@ -868,6 +876,7 @@ int xenmem_add_to_physmap(struct domain *d,
>>>>> struct xen_add_to_physmap *xatp,
>>>>>            }
>>>>>        }
>>>>>    +#ifdef CONFIG_HAS_PASSTHROUGH
>>>>>        if ( is_iommu_enabled(d) )
>>>>>        {
>>>>>            int ret;
>>>>> @@ -894,6 +903,7 @@ int xenmem_add_to_physmap(struct domain *d,
>>>>> struct xen_add_to_physmap *xatp,
>>>>>            if ( unlikely(ret) && rc >= 0 )
>>>>>                rc = ret;
>>>>>        }
>>>>> +#endif
>>>>>          return rc;
>>>>>    }
>>>>
>>>> I wonder whether all of these wouldn't better become CONFIG_X86:
>>>> ISTR Julien indicating that he doesn't see the override getting used
>>>> on Arm. (Julien, please correct me if I'm misremembering.)
>>>
>>> Right, so far, I haven't been in favor to introduce it because:
>>>     1) The P2M code may free some memory. So you can't always ignore
>>> the flush (I think this is wrong for the upper layer to know when this
>>> can happen).
>>>     2) It is unclear what happen if the IOMMU TLBs and the PT contains
>>> different mappings (I received conflicted advice).
>>>
>>> So it is better to always flush and as early as possible.
>>
>> So keep it as is or switch to CONFIG_X86?
> 
> Please switch, unless anyone else voices a strong opinion towards
> keeping as is.

I would like to avoid adding more #ifdef CONFIG_X86 in the common code. 
Can we instead provide a wrapper for them?

Cheers,

-- 
Julien Grall