linux-kernel - Re: [PATCH v3 3/3] xen: don't require virtio with grants for non-PV guests

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Date:   Wed, 22 Jun 2022 18:18:26 +0300
From:   Oleksandr <olekstysh@...il.com>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        linux-arm-kernel@...ts.infradead.org,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH v3 3/3] xen: don't require virtio with grants for non-PV
 guests


On 22.06.22 17:35, Juergen Gross wrote:


Hello Juergen

> On 22.06.22 11:03, Oleksandr wrote:
>>
>> On 22.06.22 09:38, 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.
>>>
>>> On Arm require VIRTIO_F_ACCESS_PLATFORM for devices having been listed
>>> in the device tree to use grants.
>>>
>>> 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)
>>> V3:
>>> - rebase to callback method
>>
>>
>> Patch looks good, just one NIT ...
>>
>>
>>> ---
>>>   arch/arm/xen/enlighten.c     |  4 +++-
>>>   arch/x86/xen/enlighten_hvm.c |  4 +++-
>>>   arch/x86/xen/enlighten_pv.c  |  5 ++++-
>>>   drivers/xen/Kconfig          |  9 +++++++++
>>>   drivers/xen/grant-dma-ops.c  | 10 ++++++++++
>>>   include/xen/xen-ops.h        |  6 ++++++
>>>   include/xen/xen.h            |  8 --------
>>>   7 files changed, 35 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
>>> index 1f9c3ba32833..93c8ccbf2982 100644
>>> --- a/arch/arm/xen/enlighten.c
>>> +++ b/arch/arm/xen/enlighten.c
>>> @@ -34,6 +34,7 @@
>>>   #include <linux/timekeeping.h>
>>>   #include <linux/timekeeper_internal.h>
>>>   #include <linux/acpi.h>
>>> +#include <linux/virtio_anchor.h>
>>>   #include <linux/mm.h>
>>> @@ -443,7 +444,8 @@ static int __init xen_guest_init(void)
>>>       if (!xen_domain())
>>>           return 0;
>>> -    xen_set_restricted_virtio_memory_access();
>>> +    if (IS_ENABLED(CONFIG_XEN_VIRTIO))
>>> +        virtio_set_mem_acc_cb(xen_virtio_mem_acc);
>>>       if (!acpi_disabled)
>>>           xen_acpi_guest_init();
>>> diff --git a/arch/x86/xen/enlighten_hvm.c 
>>> b/arch/x86/xen/enlighten_hvm.c
>>> index 8b71b1dd7639..28762f800596 100644
>>> --- a/arch/x86/xen/enlighten_hvm.c
>>> +++ b/arch/x86/xen/enlighten_hvm.c
>>> @@ -4,6 +4,7 @@
>>>   #include <linux/cpu.h>
>>>   #include <linux/kexec.h>
>>>   #include <linux/memblock.h>
>>> +#include <linux/virtio_anchor.h>
>>>   #include <xen/features.h>
>>>   #include <xen/events.h>
>>> @@ -195,7 +196,8 @@ static void __init xen_hvm_guest_init(void)
>>>       if (xen_pv_domain())
>>>           return;
>>> -    xen_set_restricted_virtio_memory_access();
>>> +    if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT))
>>> + virtio_set_mem_acc_cb(virtio_require_restricted_mem_acc);
>>>       init_hvm_pv_info();
>>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
>>> index e3297b15701c..5aaae8a77f55 100644
>>> --- a/arch/x86/xen/enlighten_pv.c
>>> +++ b/arch/x86/xen/enlighten_pv.c
>>> @@ -31,6 +31,7 @@
>>>   #include <linux/gfp.h>
>>>   #include <linux/edd.h>
>>>   #include <linux/reboot.h>
>>> +#include <linux/virtio_anchor.h>
>>>   #include <xen/xen.h>
>>>   #include <xen/events.h>
>>> @@ -109,7 +110,9 @@ static DEFINE_PER_CPU(struct tls_descs, 
>>> shadow_tls_desc);
>>>   static void __init xen_pv_init_platform(void)
>>>   {
>>> -    xen_set_restricted_virtio_memory_access();
>>> +    /* PV guests can't operate virtio devices without grants. */
>>> +    if (IS_ENABLED(CONFIG_XEN_VIRTIO))
>>> + virtio_set_mem_acc_cb(virtio_require_restricted_mem_acc);
>>>       populate_extra_pte(fix_to_virt(FIX_PARAVIRT_BOOTMAP));
>>> 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/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>>> index fc0142484001..8973fc1e9ccc 100644
>>> --- a/drivers/xen/grant-dma-ops.c
>>> +++ b/drivers/xen/grant-dma-ops.c
>>> @@ -12,6 +12,8 @@
>>>   #include <linux/of.h>
>>>   #include <linux/pfn.h>
>>>   #include <linux/xarray.h>
>>> +#include <linux/virtio_anchor.h>
>>> +#include <linux/virtio.h>
>>>   #include <xen/xen.h>
>>>   #include <xen/xen-ops.h>
>>>   #include <xen/grant_table.h>
>>> @@ -287,6 +289,14 @@ bool xen_is_grant_dma_device(struct device *dev)
>>>       return has_iommu;
>>>   }
>>> +bool xen_virtio_mem_acc(struct virtio_device *dev)
>>> +{
>>> +    if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT))
>>> +        return true;
>>> +
>>> +    return xen_is_grant_dma_device(dev->dev.parent);
>>> +}
>>
>>
>>     ... I am thinking would it be better to move this to 
>> xen/xen-ops.h as grant-dma-ops.c is generic (not only for virtio, 
>> although the virtio is the first use-case)
>
> I dislike using a function marked as inline in a function vector.
>
> We could add another module "xen-virtio" for this purpose, but this seems
> to be overkill.
>
> I think we should just leave it here and move it later in case more real
> virtio dependent stuff is being added.

I am happy with that explanation.

Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>


>
>
>
> Juergen

-- 
Regards,

Oleksandr Tyshchenko