Re: [Xen-devel] [PATCH v5 05/10] domain: introduce XEN_DOMCTL_CDF_iommu flag

[Julien Grall <julien.grall@arm.com>]

Hi Paul,

On 14/08/2019 15:26, Paul Durrant wrote:
>> -----Original Message-----
>> From: Julien Grall <julien.grall@arm.com>
>> Sent: 14 August 2019 15:00
>> To: Paul Durrant <Paul.Durrant@citrix.com>; xen-devel@lists.xenproject.org
>> Cc: Ian Jackson <Ian.Jackson@citrix.com>; Wei Liu <wl@xen.org>; Anthony Perard
>> <anthony.perard@citrix.com>; Andrew Cooper <Andrew.Cooper3@citrix.com>; George Dunlap
>> <George.Dunlap@citrix.com>; Jan Beulich <jbeulich@suse.com>; Konrad Rzeszutek Wilk
>> <konrad.wilk@oracle.com>; Stefano Stabellini <sstabellini@kernel.org>; Tim (Xen.org) <tim@xen.org>;
>> Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>; Roger Pau Monne <roger.pau@citrix.com>
>> Subject: Re: [PATCH v5 05/10] domain: introduce XEN_DOMCTL_CDF_iommu flag
>>
>> Hi Paul,
>>
>> On 14/08/2019 14:38, Paul Durrant wrote:
>>> This patch introduces a common domain creation flag to determine whether
>>> the domain is permitted to make use of the IOMMU. Currently the flag is
>>> always set (for both dom0 and domU) if the IOMMU is globally enabled
>>> (i.e. iommu_enabled == 1). sanitise_domain_config() is modified to reject
>>> the flag if !iommu_enabled.
>>>
>>> A new helper function, is_iommu_enabled(), is added to test the flag and
>>> iommu_domain_init() will return immediately if !is_iommu_enabled(). This is
>>> slightly different to the previous behaviour based on !iommu_enabled where
>>> the call to arch_iommu_domain_init() was made regardless, however it appears
>>> that this call was only necessary to initialize the dt_devices list for ARM
>>> such that iommu_release_dt_devices() can be called unconditionally by
>>> domain_relinquish_resources(). Adding a simple check of is_iommu_enabled()
>>> into iommu_release_dt_devices() keeps this unconditional call working.
>>>
>>> No functional change should be observed with this patch applied.
>>>
>>> Subsequent patches will allow the toolstack to control whether use of the
>>> IOMMU is enabled for a domain.
>>>
>>> NOTE: The introduction of the is_iommu_enabled() helper function might
>>>         seem excessive but its use is expected to increase with subsequent
>>>         patches. Also, having iommu_domain_init() bail before calling
>>>         arch_iommu_domain_init() is not strictly necessary, but I think the
>>>         consequent addition of the call to is_iommu_enabled() in
>>>         iommu_release_dt_devices() makes the code clearer.
>>>
>>> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
>>> ---
>>> Cc: Ian Jackson <ian.jackson@eu.citrix.com>
>>> Cc: Wei Liu <wl@xen.org>
>>> Cc: Anthony PERARD <anthony.perard@citrix.com>
>>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>>> Cc: George Dunlap <George.Dunlap@eu.citrix.com>
>>> Cc: Jan Beulich <jbeulich@suse.com>
>>> Cc: Julien Grall <julien.grall@arm.com>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>> Cc: Stefano Stabellini <sstabellini@kernel.org>
>>> Cc: Tim Deegan <tim@xen.org>
>>> Cc: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
>>> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
>>>
>>> Previously part of series https://lists.xenproject.org/archives/html/xen-devel/2019-07/msg02267.html
>>>
>>> v5:
>>>    - Move is_iommu_enabled() check into iommu_domain_init()
>>>    - Reject XEN_DOMCTL_CDF_iommu in sanitise_domain_config() if !iommu_enabled
>>>    - Use evaluate_nospec() in defintion of is_iommu_enabled()
>>> ---
>>>    tools/libxl/libxl_create.c            | 8 ++++++++
>>>    xen/arch/arm/domain.c                 | 1 -
>>>    xen/arch/arm/setup.c                  | 3 +++
>>>    xen/arch/x86/setup.c                  | 3 +++
>>>    xen/common/domain.c                   | 9 ++++++++-
>>>    xen/drivers/passthrough/device_tree.c | 3 +++
>>>    xen/drivers/passthrough/iommu.c       | 6 +++---
>>>    xen/include/public/domctl.h           | 4 ++++
>>>    xen/include/xen/sched.h               | 5 +++++
>>>    9 files changed, 37 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
>>> index 03ce166f4f..050ef042cd 100644
>>> --- a/tools/libxl/libxl_create.c
>>> +++ b/tools/libxl/libxl_create.c
>>> @@ -555,6 +555,7 @@ int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
>>>                .max_grant_frames = b_info->max_grant_frames,
>>>                .max_maptrack_frames = b_info->max_maptrack_frames,
>>>            };
>>> +        libxl_physinfo physinfo;
>>>
>>>            if (info->type != LIBXL_DOMAIN_TYPE_PV) {
>>>                create.flags |= XEN_DOMCTL_CDF_hvm_guest;
>>> @@ -564,6 +565,13 @@ int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
>>>                    libxl_defbool_val(info->oos) ? 0 : XEN_DOMCTL_CDF_oos_off;
>>>            }
>>>
>>> +        rc = libxl_get_physinfo(ctx, &physinfo);
>>> +        if (rc < 0)
>>> +            goto out;
>>> +
>>> +        if (physinfo.cap_hvm_directio)
>>> +            create.flags |= XEN_DOMCTL_CDF_iommu;
>>
>> This is not going to work well on Arm as XEN_SYSCTL_PHYSCAP_directio is never set.
> 
> Oh, that's true. I need to pull that into the common sysctl handler. It also looks like XEN_SYSCTL_PHYSCAP_hvm should always be set for too, but ARMs arch_do_physinfo() is completely empty at the moment.

Looking at the header (include/public/sysctl.h) it is pretty clear that the two 
PHYSCAP are x86 specific.

Even if Arm guests are based on the same concept as HVM/PVH guest. They are just 
called "Arm guest". We have always taken action so far to say they are HVM/PVH 
guest when presenting to the user. For instance, we strongly recommend to have 
the guest type in your configuration.

So I am not entirely sure we want to expose the capability for Arm. Otherwise a 
user may think it is fine to use "hvm" guest type.

Cheers,

-- 
Julien Grall

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