Re: [Xen-devel] [PATCH v13 0/4] add per-domain IOMMU control

[Julien Grall <Julien.Grall@xxxxxxx>]

Hi,

On 25/09/2019 17:10, Paul Durrant wrote:
>> -----Original Message-----
>> From: Oleksandr <olekstysh@xxxxxxxxx>
>> Sent: 25 September 2019 16:50
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Jan Beulich' <jbeulich@xxxxxxxx>
>> Cc: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>; Stefano Stabellini 
>> <sstabellini@xxxxxxxxxx>; Wei Liu
>> <wl@xxxxxxx>; KonradRzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Andrew Cooper
>> <Andrew.Cooper3@xxxxxxxxxx>; David Scott <dave@xxxxxxxxxx>; Tim (Xen.org) 
>> <tim@xxxxxxx>; George Dunlap
>> <George.Dunlap@xxxxxxxxxx>; Tamas K Lengyel <tamas@xxxxxxxxxxxxx>; Ian 
>> Jackson
>> <Ian.Jackson@xxxxxxxxxx>; Anthony Perard <anthony.perard@xxxxxxxxxx>; 
>> xen-devel@xxxxxxxxxxxxxxxxxxxx;
>> Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; Roger Pau Monne 
>> <roger.pau@xxxxxxxxxx>; Julien Grall
>> <julien.grall@xxxxxxx>
>> Subject: Re: [Xen-devel] [PATCH v13 0/4] add per-domain IOMMU control
>>
>>
>> [CC Julien]
>>
>>
>> Hi Paul
>>
>> I may mistake, but looks like
>>
>> 80ff3d338dc93260b41ffeeebb0f852c2edef9ce iommu: tidy up
>> iommu_use_hap_pt() and need_iommu_pt_sync() macros
>>
>> triggers ASSERT_UNREACHABLE on Arm if no IOMMU has been found (I built
>> with my platform's IOMMU driver disabled: # CONFIG_IPMMU_VMSA is not set) .
>>
>> So, iommu_setup() calls clear_iommu_hap_pt_share() with
>> iommu_hap_pt_share being set (CONFIG_IOMMU_FORCE_PT_SHARE=y) which,
>> actually, triggers ASSERT.
>>
> 
> Here a minimal patch, leaving 'force pt share' in place. Does this avoid the 
> problem?
> 
> ---8<---
> diff --git a/xen/common/sysctl.c b/xen/common/sysctl.c
> index e8763c7fdf..f88a285e7f 100644
> --- a/xen/common/sysctl.c
> +++ b/xen/common/sysctl.c
> @@ -268,9 +268,11 @@ long do_sysctl(XEN_GUEST_HANDLE_PARAM(xen_sysctl_t) 
> u_sysctl)
>           pi->max_mfn = get_upper_mfn_bound();
>           arch_do_physinfo(pi);
>           if ( iommu_enabled )
> +        {
>               pi->capabilities |= XEN_SYSCTL_PHYSCAP_directio;
> -        if ( iommu_hap_pt_share )
> -            pi->capabilities |= XEN_SYSCTL_PHYSCAP_iommu_hap_pt_share;
> +            if ( iommu_hap_pt_share )
> +                pi->capabilities |= XEN_SYSCTL_PHYSCAP_iommu_hap_pt_share;
> +        }
> 
>           if ( copy_to_guest(u_sysctl, op, 1) )
>               ret = -EFAULT;
> diff --git a/xen/include/xen/iommu.h b/xen/include/xen/iommu.h
> index 7c3003f3f1..6a10a24128 100644
> --- a/xen/include/xen/iommu.h
> +++ b/xen/include/xen/iommu.h
> @@ -68,8 +68,6 @@ static inline void clear_iommu_hap_pt_share(void)
>   {
>   #ifndef iommu_hap_pt_share
>       iommu_hap_pt_share = false;
> -#elif iommu_hap_pt_share
> -    ASSERT_UNREACHABLE();
>   #endif

IHMO, calling this function is a mistake on platform only supporting 
shared page-table so the ASSERT() should be kept here.

This raises the question why the function is actually called from common 
code. iommu_hap_enabled() should technically not be used in any code if 
the IOMMU is not enabled/present. So what are you trying to prevent here?

Cheers,

-- 
Julien Grall
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