linux-kernel - Re: [PATCH v2] xen: don't require virtio with grants for non-PV guests

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Date:   Fri, 17 Jun 2022 07:41:39 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr <olekstysh@...il.com>, hch@...radead.org,
        Stefano Stabellini <sstabellini@...nel.org>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        viresh.kumar@...aro.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH v2] xen: don't require virtio with grants for non-PV
 guests

On 16.06.22 22:33, Oleksandr wrote:
> 
> On 16.06.22 11:56, Juergen Gross wrote:
> 
> Hello Juergen, all
> 
> 
>> On 16.06.22 09:31, Oleksandr wrote:
>>>
>>> On 16.06.22 08:37, Juergen Gross wrote:
>>>
>>>
>>> Hello Juergen
>>>
>>>> Commit fa1f57421e0b ("xen/virtio: Enable restricted memory access using
>>>> Xen grant mappings") introduced a new requirement for using virtio
>>>> devices: the backend now needs to support the VIRTIO_F_ACCESS_PLATFORM
>>>> feature.
>>>>
>>>> This is an undue requirement for non-PV guests, as those can be operated
>>>> with existing backends without any problem, as long as those backends
>>>> are running in dom0.
>>>>
>>>> Per default allow virtio devices without grant support for non-PV
>>>> guests.
>>>>
>>>> Add a new config item to always force use of grants for virtio.
>>>>
>>>> Fixes: fa1f57421e0b ("xen/virtio: Enable restricted memory access using Xen 
>>>> grant mappings")
>>>> Reported-by: Viresh Kumar <viresh.kumar@...aro.org>
>>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>>> ---
>>>> V2:
>>>> - remove command line parameter (Christoph Hellwig)
>>>> ---
>>>>   drivers/xen/Kconfig | 9 +++++++++
>>>>   include/xen/xen.h   | 2 +-
>>>>   2 files changed, 10 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
>>>> index bfd5f4f706bc..a65bd92121a5 100644
>>>> --- a/drivers/xen/Kconfig
>>>> +++ b/drivers/xen/Kconfig
>>>> @@ -355,4 +355,13 @@ config XEN_VIRTIO
>>>>         If in doubt, say n.
>>>> +config XEN_VIRTIO_FORCE_GRANT
>>>> +    bool "Require Xen virtio support to use grants"
>>>> +    depends on XEN_VIRTIO
>>>> +    help
>>>> +      Require virtio for Xen guests to use grant mappings.
>>>> +      This will avoid the need to give the backend the right to map all
>>>> +      of the guest memory. This will need support on the backend side
>>>> +      (e.g. qemu or kernel, depending on the virtio device types used).
>>>> +
>>>>   endmenu
>>>> diff --git a/include/xen/xen.h b/include/xen/xen.h
>>>> index 0780a81e140d..4d4188f20337 100644
>>>> --- a/include/xen/xen.h
>>>> +++ b/include/xen/xen.h
>>>> @@ -56,7 +56,7 @@ extern u64 xen_saved_max_mem_size;
>>>>   static inline void xen_set_restricted_virtio_memory_access(void)
>>>>   {
>>>> -    if (IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_domain())
>>>> +    if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || xen_pv_domain())
>>>>           platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
>>>
>>>
>>> Looks like, the flag will be *always* set for paravirtualized guests even if 
>>> CONFIG_XEN_VIRTIO disabled.
>>>
>>> Maybe we should clarify the check?
>>>
>>>
>>> if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || 
>>> IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_pv_domain())
>>>
>>>      platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
>>>
>>
>> Yes, we should. I had the function in grant-dma-ops.c in V1, and could drop the
>> CONFIG_XEN_VIRTIO dependency for that reason.
>>
>> I'll wait for more comments before sending V3, though.
> 
> ok
> 
> 
> 
> Please note, I am happy with current patch and it works in my Arm64 based 
> environment.
> 
> Just one moment to consider.
> 
> 
> As it was already mentioned earlier in current thread the 
> PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS (former 
> arch_has_restricted_virtio_memory_access()) is not per device but about the 
> whole guest. Being set it makes VIRTIO_F_ACCESS_PLATFORM and VIRTIO_F_VERSION_1 
> features mandatory for *all* virtio devices in the guest.
> 
> The question is “Do we want/need to lift this restriction for some devices 
> (which backends are trusted so can access all guest memory) at the same time”? 

No, if you need some virtio devices to not use grants, then don't set
PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS.

Please see my answer to Stefano's alternative solution for my idea how to
resolve this via a per-device setting.


Juergen

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