[Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

[Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

[Alexander Graf]

ARM is amazing when it comes to cache coherency and VMs. While any sane
architecture allows the host to override the guest's caching attributes,
that's very hard to do on ARM.

That means that the guest may directly access guest memory bypassing the
cache while QEMU happily writes to / reads from cache. The end result is
very nasty, because both sides see very different views of the world.
 
That means that we need to be very cautious to tell guests that devices
that QEMU emulates are going to use data in the cache rather than directly
on memory.

We added this to PCI a while back for DT (5d636e21 "hw/arm/virt: mark the PCIe
host controller as DMA coherent in the DT") and ACPI (bc64b96 "hw/arm/virt-
acpi-build: _CCA attribute is compulsory") but never updated virtio-mmio or
fw-cfg in DT or ACPI tables.

This patch set adds the respective cache coherency flags for them in both DT and
ACPI.

Fortunately, no guests except for Linux 4.9.7 and 4.9.8 are broken because of
this. Upstream realized quickly enough that every user of virtio-mmio out there
describes its cache coherency incorrectly and reverted the patch that would
require said dma coherency flag. But we should be safe for the future and "do
the right thing".

Alexander Graf (4):
  target-arm: Declare virtio-mmio as dma-coherent in dt
  hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI
  hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI
  hw/arm/virt: Declare fwcfg as dma cache coherent in dt

 hw/arm/vexpress.c        | 1 +
 hw/arm/virt-acpi-build.c | 2 ++
 hw/arm/virt.c            | 2 ++
 3 files changed, 5 insertions(+)

-- 
2.10.0

[Qemu-devel] [PATCH v2 1/4] target-arm: Declare virtio-mmio as dma-coherent in dt

[Alexander Graf]

QEMU emulated hardware is always dma coherent with its guest. We do
annotate that correctly on the PCI host controller, but left out
virtio-mmio.

Recent kernels have started to interpret that flag rather than take
dma coherency as granted with virtio-mmio. While that is considered
a kernel bug, as it breaks previously working systems, it showed that
our dt description is incomplete.

This patch adds the respective marker that allows guest OSs to evaluate
that our virtio-mmio devices are indeed cache coherent.

Signed-off-by: Alexander Graf <agraf@suse.de>
---
 hw/arm/vexpress.c | 1 +
 hw/arm/virt.c     | 1 +
 2 files changed, 2 insertions(+)

diff --git a/hw/arm/vexpress.c b/hw/arm/vexpress.c
index 58760f4..e057568 100644
--- a/hw/arm/vexpress.c
+++ b/hw/arm/vexpress.c
@@ -452,6 +452,7 @@ static int add_virtio_mmio_node(void *fdt, uint32_t acells, uint32_t scells,
                                        acells, addr, scells, size);
     qemu_fdt_setprop_cells(fdt, nodename, "interrupt-parent", intc);
     qemu_fdt_setprop_cells(fdt, nodename, "interrupts", 0, irq, 1);
+    qemu_fdt_setprop(fdt, nodename, "dma-coherent", NULL, 0);
     g_free(nodename);
     if (rc) {
         return -1;
diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 1f216cf..14881fa 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -797,6 +797,7 @@ static void create_virtio_devices(const VirtMachineState *vms, qemu_irq *pic)
         qemu_fdt_setprop_cells(vms->fdt, nodename, "interrupts",
                                GIC_FDT_IRQ_TYPE_SPI, irq,
                                GIC_FDT_IRQ_FLAGS_EDGE_LO_HI);
+        qemu_fdt_setprop(vms->fdt, nodename, "dma-coherent", NULL, 0);
         g_free(nodename);
     }
 }
-- 
2.10.0

[Qemu-devel] [PATCH v2 2/4] hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI

[Alexander Graf]

Virtio-mmio devices can directly access guest memory and do so in cache
coherent fashion. Tell the guest about that fact when it's using ACPI.

Signed-off-by: Alexander Graf <agraf@suse.de>
---
 hw/arm/virt-acpi-build.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
index 07a10ac..8955a9d 100644
--- a/hw/arm/virt-acpi-build.c
+++ b/hw/arm/virt-acpi-build.c
@@ -135,6 +135,7 @@ static void acpi_dsdt_add_virtio(Aml *scope,
         Aml *dev = aml_device("VR%02u", i);
         aml_append(dev, aml_name_decl("_HID", aml_string("LNRO0005")));
         aml_append(dev, aml_name_decl("_UID", aml_int(i)));
+        aml_append(dev, aml_name_decl("_CCA", aml_int(1)));
 
         Aml *crs = aml_resource_template();
         aml_append(crs, aml_memory32_fixed(base, size, AML_READ_WRITE));
-- 
2.10.0

Re: [Qemu-devel] [PATCH v2 2/4] hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI

[Shannon Zhao]

On 2017/2/9 20:53, Alexander Graf wrote:
> Virtio-mmio devices can directly access guest memory and do so in cache
> coherent fashion. Tell the guest about that fact when it's using ACPI.
> 
> Signed-off-by: Alexander Graf <agraf@suse.de>
> ---
>  hw/arm/virt-acpi-build.c | 1 +
>  1 file changed, 1 insertion(+)
> 
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>

> diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> index 07a10ac..8955a9d 100644
> --- a/hw/arm/virt-acpi-build.c
> +++ b/hw/arm/virt-acpi-build.c
> @@ -135,6 +135,7 @@ static void acpi_dsdt_add_virtio(Aml *scope,
>          Aml *dev = aml_device("VR%02u", i);
>          aml_append(dev, aml_name_decl("_HID", aml_string("LNRO0005")));
>          aml_append(dev, aml_name_decl("_UID", aml_int(i)));
> +        aml_append(dev, aml_name_decl("_CCA", aml_int(1)));
>  
>          Aml *crs = aml_resource_template();
>          aml_append(crs, aml_memory32_fixed(base, size, AML_READ_WRITE));
> 

-- 
Shannon

[Qemu-devel] [PATCH v2 3/4] hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI

[Alexander Graf]

Fw-cfg recently learned how to directly access guest memory and does so in
cache coherent fashion. Tell the guest about that fact when it's using ACPI.

Signed-off-by: Alexander Graf <agraf@suse.de>
---
 hw/arm/virt-acpi-build.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
index 8955a9d..0835e59 100644
--- a/hw/arm/virt-acpi-build.c
+++ b/hw/arm/virt-acpi-build.c
@@ -90,6 +90,7 @@ static void acpi_dsdt_add_fw_cfg(Aml *scope, const MemMapEntry *fw_cfg_memmap)
     aml_append(dev, aml_name_decl("_HID", aml_string("QEMU0002")));
     /* device present, functioning, decoding, not shown in UI */
     aml_append(dev, aml_name_decl("_STA", aml_int(0xB)));
+    aml_append(dev, aml_name_decl("_CCA", aml_int(1)));
 
     Aml *crs = aml_resource_template();
     aml_append(crs, aml_memory32_fixed(fw_cfg_memmap->base,
-- 
2.10.0

Re: [Qemu-devel] [PATCH v2 3/4] hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI

[Shannon Zhao]

On 2017/2/9 20:53, Alexander Graf wrote:
> Fw-cfg recently learned how to directly access guest memory and does so in
> cache coherent fashion. Tell the guest about that fact when it's using ACPI.
> 
> Signed-off-by: Alexander Graf <agraf@suse.de>
> ---
>  hw/arm/virt-acpi-build.c | 1 +
>  1 file changed, 1 insertion(+)
> 
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>

> diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> index 8955a9d..0835e59 100644
> --- a/hw/arm/virt-acpi-build.c
> +++ b/hw/arm/virt-acpi-build.c
> @@ -90,6 +90,7 @@ static void acpi_dsdt_add_fw_cfg(Aml *scope, const MemMapEntry *fw_cfg_memmap)
>      aml_append(dev, aml_name_decl("_HID", aml_string("QEMU0002")));
>      /* device present, functioning, decoding, not shown in UI */
>      aml_append(dev, aml_name_decl("_STA", aml_int(0xB)));
> +    aml_append(dev, aml_name_decl("_CCA", aml_int(1)));
>  
>      Aml *crs = aml_resource_template();
>      aml_append(crs, aml_memory32_fixed(fw_cfg_memmap->base,
> 

-- 
Shannon

[Qemu-devel] [PATCH v2 4/4] hw/arm/virt: Declare fwcfg as dma cache coherent in dt

[Alexander Graf]

Fw-cfg recently learned how to directly access guest memory and does so in
cache coherent fashion. Tell the guest about that fact when it's using DT.

Signed-off-by: Alexander Graf <agraf@suse.de>
---
 hw/arm/virt.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 14881fa..da9622f 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -929,6 +929,7 @@ static FWCfgState *create_fw_cfg(const VirtMachineState *vms, AddressSpace *as)
                             "compatible", "qemu,fw-cfg-mmio");
     qemu_fdt_setprop_sized_cells(vms->fdt, nodename, "reg",
                                  2, base, 2, size);
+    qemu_fdt_setprop(vms->fdt, nodename, "dma-coherent", NULL, 0);
     g_free(nodename);
     return fw_cfg;
 }
-- 
2.10.0

Re: [Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

[Laszlo Ersek]

On 02/09/17 13:53, Alexander Graf wrote:
> ARM is amazing when it comes to cache coherency and VMs. While any sane
> architecture allows the host to override the guest's caching attributes,
> that's very hard to do on ARM.
> 
> That means that the guest may directly access guest memory bypassing the
> cache while QEMU happily writes to / reads from cache. The end result is
> very nasty, because both sides see very different views of the world.
>  
> That means that we need to be very cautious to tell guests that devices
> that QEMU emulates are going to use data in the cache rather than directly
> on memory.
> 
> We added this to PCI a while back for DT (5d636e21 "hw/arm/virt: mark the PCIe
> host controller as DMA coherent in the DT") and ACPI (bc64b96 "hw/arm/virt-
> acpi-build: _CCA attribute is compulsory") but never updated virtio-mmio or
> fw-cfg in DT or ACPI tables.
> 
> This patch set adds the respective cache coherency flags for them in both DT and
> ACPI.
> 
> Fortunately, no guests except for Linux 4.9.7 and 4.9.8 are broken because of
> this. Upstream realized quickly enough that every user of virtio-mmio out there
> describes its cache coherency incorrectly and reverted the patch that would
> require said dma coherency flag. But we should be safe for the future and "do
> the right thing".
> 
> Alexander Graf (4):
>   target-arm: Declare virtio-mmio as dma-coherent in dt
>   hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI
>   hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI
>   hw/arm/virt: Declare fwcfg as dma cache coherent in dt
> 
>  hw/arm/vexpress.c        | 1 +
>  hw/arm/virt-acpi-build.c | 2 ++
>  hw/arm/virt.c            | 2 ++
>  3 files changed, 5 insertions(+)
> 

Famous last words:
series
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Should we replicate patch #3 to QEMU0002 / FWCF in
"hw/i386/acpi-build.c" too? Or is it that we couldn't care less about
_CCA on x86? :) (Can't really muster the energy right now to look it up
in the ACPI spec, sorry!)

Thanks
Laszlo

Re: [Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

[Michael S. Tsirkin]

On Thu, Feb 09, 2017 at 02:15:36PM +0100, Laszlo Ersek wrote:
> On 02/09/17 13:53, Alexander Graf wrote:
> > ARM is amazing when it comes to cache coherency and VMs. While any sane
> > architecture allows the host to override the guest's caching attributes,
> > that's very hard to do on ARM.
> > 
> > That means that the guest may directly access guest memory bypassing the
> > cache while QEMU happily writes to / reads from cache. The end result is
> > very nasty, because both sides see very different views of the world.
> >  
> > That means that we need to be very cautious to tell guests that devices
> > that QEMU emulates are going to use data in the cache rather than directly
> > on memory.
> > 
> > We added this to PCI a while back for DT (5d636e21 "hw/arm/virt: mark the PCIe
> > host controller as DMA coherent in the DT") and ACPI (bc64b96 "hw/arm/virt-
> > acpi-build: _CCA attribute is compulsory") but never updated virtio-mmio or
> > fw-cfg in DT or ACPI tables.
> > 
> > This patch set adds the respective cache coherency flags for them in both DT and
> > ACPI.
> > 
> > Fortunately, no guests except for Linux 4.9.7 and 4.9.8 are broken because of
> > this. Upstream realized quickly enough that every user of virtio-mmio out there
> > describes its cache coherency incorrectly and reverted the patch that would
> > require said dma coherency flag. But we should be safe for the future and "do
> > the right thing".
> > 
> > Alexander Graf (4):
> >   target-arm: Declare virtio-mmio as dma-coherent in dt
> >   hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI
> >   hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI
> >   hw/arm/virt: Declare fwcfg as dma cache coherent in dt
> > 
> >  hw/arm/vexpress.c        | 1 +
> >  hw/arm/virt-acpi-build.c | 2 ++
> >  hw/arm/virt.c            | 2 ++
> >  3 files changed, 5 insertions(+)
> > 
> 
> Famous last words:
> series
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
> 
> Should we replicate patch #3 to QEMU0002 / FWCF in
> "hw/i386/acpi-build.c" too? Or is it that we couldn't care less about
> _CCA on x86? :) (Can't really muster the energy right now to look it up
> in the ACPI spec, sorry!)
> 
> Thanks
> Laszlo

ACPI spec says:
On platforms for which existing default cache-coherency behavior of the OS is not adequate, _CCA
enables the OS to adapt to the differences

So I think we don't need it on x86.

-- 
MST

Re: [Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

[Alexander Graf]

> Am 09.02.2017 um 19:13 schrieb Michael S. Tsirkin <mst@redhat.com>:
> 
>> On Thu, Feb 09, 2017 at 02:15:36PM +0100, Laszlo Ersek wrote:
>>> On 02/09/17 13:53, Alexander Graf wrote:
>>> ARM is amazing when it comes to cache coherency and VMs. While any sane
>>> architecture allows the host to override the guest's caching attributes,
>>> that's very hard to do on ARM.
>>> 
>>> That means that the guest may directly access guest memory bypassing the
>>> cache while QEMU happily writes to / reads from cache. The end result is
>>> very nasty, because both sides see very different views of the world.
>>> 
>>> That means that we need to be very cautious to tell guests that devices
>>> that QEMU emulates are going to use data in the cache rather than directly
>>> on memory.
>>> 
>>> We added this to PCI a while back for DT (5d636e21 "hw/arm/virt: mark the PCIe
>>> host controller as DMA coherent in the DT") and ACPI (bc64b96 "hw/arm/virt-
>>> acpi-build: _CCA attribute is compulsory") but never updated virtio-mmio or
>>> fw-cfg in DT or ACPI tables.
>>> 
>>> This patch set adds the respective cache coherency flags for them in both DT and
>>> ACPI.
>>> 
>>> Fortunately, no guests except for Linux 4.9.7 and 4.9.8 are broken because of
>>> this. Upstream realized quickly enough that every user of virtio-mmio out there
>>> describes its cache coherency incorrectly and reverted the patch that would
>>> require said dma coherency flag. But we should be safe for the future and "do
>>> the right thing".
>>> 
>>> Alexander Graf (4):
>>>  target-arm: Declare virtio-mmio as dma-coherent in dt
>>>  hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI
>>>  hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI
>>>  hw/arm/virt: Declare fwcfg as dma cache coherent in dt
>>> 
>>> hw/arm/vexpress.c        | 1 +
>>> hw/arm/virt-acpi-build.c | 2 ++
>>> hw/arm/virt.c            | 2 ++
>>> 3 files changed, 5 insertions(+)
>>> 
>> 
>> Famous last words:
>> series
>> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
>> 
>> Should we replicate patch #3 to QEMU0002 / FWCF in
>> "hw/i386/acpi-build.c" too? Or is it that we couldn't care less about
>> _CCA on x86? :) (Can't really muster the energy right now to look it up
>> in the ACPI spec, sorry!)
>> 
>> Thanks
>> Laszlo
> 
> ACPI spec says:
> On platforms for which existing default cache-coherency behavior of the OS is not adequate, _CCA
> enables the OS to adapt to the differences
> 
> So I think we don't need it on x86.

According to acpi 6.1, x86 explicitly defaults to dma coherent if _CCA is omitted. It's only illegal for ARM.

Alex

Re: [Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

[Laszlo Ersek]

On 02/09/17 19:27, Alexander Graf wrote:
> 
> 
>> Am 09.02.2017 um 19:13 schrieb Michael S. Tsirkin <mst@redhat.com>:
>>
>>> On Thu, Feb 09, 2017 at 02:15:36PM +0100, Laszlo Ersek wrote:
>>>> On 02/09/17 13:53, Alexander Graf wrote:
>>>> ARM is amazing when it comes to cache coherency and VMs. While any sane
>>>> architecture allows the host to override the guest's caching attributes,
>>>> that's very hard to do on ARM.
>>>>
>>>> That means that the guest may directly access guest memory bypassing the
>>>> cache while QEMU happily writes to / reads from cache. The end result is
>>>> very nasty, because both sides see very different views of the world.
>>>>
>>>> That means that we need to be very cautious to tell guests that devices
>>>> that QEMU emulates are going to use data in the cache rather than directly
>>>> on memory.
>>>>
>>>> We added this to PCI a while back for DT (5d636e21 "hw/arm/virt: mark the PCIe
>>>> host controller as DMA coherent in the DT") and ACPI (bc64b96 "hw/arm/virt-
>>>> acpi-build: _CCA attribute is compulsory") but never updated virtio-mmio or
>>>> fw-cfg in DT or ACPI tables.
>>>>
>>>> This patch set adds the respective cache coherency flags for them in both DT and
>>>> ACPI.
>>>>
>>>> Fortunately, no guests except for Linux 4.9.7 and 4.9.8 are broken because of
>>>> this. Upstream realized quickly enough that every user of virtio-mmio out there
>>>> describes its cache coherency incorrectly and reverted the patch that would
>>>> require said dma coherency flag. But we should be safe for the future and "do
>>>> the right thing".
>>>>
>>>> Alexander Graf (4):
>>>>  target-arm: Declare virtio-mmio as dma-coherent in dt
>>>>  hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI
>>>>  hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI
>>>>  hw/arm/virt: Declare fwcfg as dma cache coherent in dt
>>>>
>>>> hw/arm/vexpress.c        | 1 +
>>>> hw/arm/virt-acpi-build.c | 2 ++
>>>> hw/arm/virt.c            | 2 ++
>>>> 3 files changed, 5 insertions(+)
>>>>
>>>
>>> Famous last words:
>>> series
>>> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
>>>
>>> Should we replicate patch #3 to QEMU0002 / FWCF in
>>> "hw/i386/acpi-build.c" too? Or is it that we couldn't care less about
>>> _CCA on x86? :) (Can't really muster the energy right now to look it up
>>> in the ACPI spec, sorry!)
>>>
>>> Thanks
>>> Laszlo
>>
>> ACPI spec says:
>> On platforms for which existing default cache-coherency behavior of the OS is not adequate, _CCA
>> enables the OS to adapt to the differences
>>
>> So I think we don't need it on x86.
> 
> According to acpi 6.1, x86 explicitly defaults to dma coherent if _CCA is omitted. It's only illegal for ARM.

Incredible; a finding that, for a change, does not create more work.

Thanks.
Laszlo

Re: [Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

[Ard Biesheuvel]

On 9 February 2017 at 12:53, Alexander Graf <agraf@suse.de> wrote:
> ARM is amazing when it comes to cache coherency and VMs. While any sane
> architecture allows the host to override the guest's caching attributes,
> that's very hard to do on ARM.
>
> That means that the guest may directly access guest memory bypassing the
> cache while QEMU happily writes to / reads from cache. The end result is
> very nasty, because both sides see very different views of the world.
>
> That means that we need to be very cautious to tell guests that devices
> that QEMU emulates are going to use data in the cache rather than directly
> on memory.
>
> We added this to PCI a while back for DT (5d636e21 "hw/arm/virt: mark the PCIe
> host controller as DMA coherent in the DT") and ACPI (bc64b96 "hw/arm/virt-
> acpi-build: _CCA attribute is compulsory") but never updated virtio-mmio or
> fw-cfg in DT or ACPI tables.
>
> This patch set adds the respective cache coherency flags for them in both DT and
> ACPI.
>
> Fortunately, no guests except for Linux 4.9.7 and 4.9.8 are broken because of
> this. Upstream realized quickly enough that every user of virtio-mmio out there
> describes its cache coherency incorrectly and reverted the patch that would
> require said dma coherency flag. But we should be safe for the future and "do
> the right thing".
>
> Alexander Graf (4):
>   target-arm: Declare virtio-mmio as dma-coherent in dt
>   hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI
>   hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI
>   hw/arm/virt: Declare fwcfg as dma cache coherent in dt
>

This looks correct to me.

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Re: [Qemu-devel] [Qemu-arm] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

[Peter Maydell]

On 9 February 2017 at 12:53, Alexander Graf <agraf@suse.de> wrote:
> ARM is amazing when it comes to cache coherency and VMs. While any sane
> architecture allows the host to override the guest's caching attributes,
> that's very hard to do on ARM.
>
> That means that the guest may directly access guest memory bypassing the
> cache while QEMU happily writes to / reads from cache. The end result is
> very nasty, because both sides see very different views of the world.
>
> That means that we need to be very cautious to tell guests that devices
> that QEMU emulates are going to use data in the cache rather than directly
> on memory.
>
> We added this to PCI a while back for DT (5d636e21 "hw/arm/virt: mark the PCIe
> host controller as DMA coherent in the DT") and ACPI (bc64b96 "hw/arm/virt-
> acpi-build: _CCA attribute is compulsory") but never updated virtio-mmio or
> fw-cfg in DT or ACPI tables.
>
> This patch set adds the respective cache coherency flags for them in both DT and
> ACPI.
>
> Fortunately, no guests except for Linux 4.9.7 and 4.9.8 are broken because of
> this. Upstream realized quickly enough that every user of virtio-mmio out there
> describes its cache coherency incorrectly and reverted the patch that would
> require said dma coherency flag. But we should be safe for the future and "do
> the right thing".
>
> Alexander Graf (4):
>   target-arm: Declare virtio-mmio as dma-coherent in dt
>   hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI
>   hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI
>   hw/arm/virt: Declare fwcfg as dma cache coherent in dt
>
>  hw/arm/vexpress.c        | 1 +
>  hw/arm/virt-acpi-build.c | 2 ++
>  hw/arm/virt.c            | 2 ++
>  3 files changed, 5 insertions(+)

The patches in this series have more lines of Reviewed-by: tags than
they do actual code changes :-)

Thanks, applied to target-arm.next.

-- PMM