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 16:35:22 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr <olekstysh@...il.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 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.


Juergen

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