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Re: [PATCH v2 3/3] [FUTURE] xen/arm: enable vPCI for domUs
Hi,
On 07/07/2023 14:13, Roger Pau Monné wrote:
On Fri, Jul 07, 2023 at 01:09:40PM +0100, Julien Grall wrote:
Hi,
On 07/07/2023 12:34, Roger Pau Monné wrote:
On Fri, Jul 07, 2023 at 12:16:46PM +0100, Julien Grall wrote:
On 07/07/2023 11:47, Roger Pau Monné wrote:
On Fri, Jul 07, 2023 at 11:33:14AM +0100, Julien Grall wrote:
Hi,
On 07/07/2023 11:06, Roger Pau Monné wrote:
On Fri, Jul 07, 2023 at 10:00:51AM +0100, Julien Grall wrote:
On 07/07/2023 02:47, Stewart Hildebrand wrote:
Note that CONFIG_HAS_VPCI_GUEST_SUPPORT is not currently used in the upstream
code base. It will be used by the vPCI series [1]. This patch is intended to be
merged as part of the vPCI series.
v1->v2:
* new patch
---
xen/arch/arm/Kconfig | 1 +
xen/arch/arm/include/asm/domain.h | 2 +-
2 files changed, 2 insertions(+), 1 deletion(-)
diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
index 4e0cc421ad48..75dfa2f5a82d 100644
--- a/xen/arch/arm/Kconfig
+++ b/xen/arch/arm/Kconfig
@@ -195,6 +195,7 @@ config PCI_PASSTHROUGH
depends on ARM_64
select HAS_PCI
select HAS_VPCI
+ select HAS_VPCI_GUEST_SUPPORT
default n
help
This option enables PCI device passthrough
diff --git a/xen/arch/arm/include/asm/domain.h
b/xen/arch/arm/include/asm/domain.h
index 1a13965a26b8..6e016b00bae1 100644
--- a/xen/arch/arm/include/asm/domain.h
+++ b/xen/arch/arm/include/asm/domain.h
@@ -298,7 +298,7 @@ static inline void arch_vcpu_block(struct vcpu *v) {}
#define arch_vm_assist_valid_mask(d) (1UL <<
VMASST_TYPE_runstate_update_flag)
-#define has_vpci(d) ({ IS_ENABLED(CONFIG_HAS_VPCI) && is_hardware_domain(d); })
+#define has_vpci(d) ({ (void)(d); IS_ENABLED(CONFIG_HAS_VPCI); })
As I mentioned in the previous patch, wouldn't this enable vPCI
unconditionally for all the domain? Shouldn't this be instead an optional
feature which would be selected by the toolstack?
I do think so, at least on x86 we signal whether vPCI should be
enabled for a domain using xen_arch_domainconfig at domain creation.
Ideally we would like to do this on a per-device basis for domUs, so
we should consider adding a new flag to xen_domctl_assign_device in
order to signal whether the assigned device should use vPCI.
I am a bit confused with this paragraph. If the device is not using vPCI,
how will it be exposed to the domain? Are you planning to support both vPCI
and PV PCI passthrough for a same domain?
You could have an external device model handling it using the ioreq
interface, like we currently do passthrough for HVM guests.
IMHO, if one decide to use QEMU for emulating the host bridge, then there is
limited point to also ask Xen to emulate the hostbridge for some other
device. So what would be the use case where you would want to be a
per-device basis decision?
You could also emulate the bridge in Xen and then have QEMU and
vPCI handle accesses to the PCI config space for different devices.
The ioreq interface already allows registering for config space
accesses on a per SBDF basis.
XenServer currently has a use-case where generic PCI device
passthrough is handled by QEMU, while some GPUs are passed through
using a custom emulator. So some domains effectively end with a QEMU
instance and a custom emulator, I don't see why you couldn't
technically replace QEMU with vPCI in this scenario.
The PCI root complex might be emulated by QEMU, or ideally by Xen.
That shouldn't prevent other device models from handling accesses for
devices, as long as accesses to the ECAM region(s) are trapped and
decoded by Xen. IOW: if we want bridges to be emulated by ioreq
servers we need to introduce an hypercall to register ECAM regions
with Xen so that it can decode accesses and forward them
appropriately.
Thanks for the clarification. Going back to the original discussion. Even
with this setup, I think we still need to tell at domain creation whether
vPCI will be used (think PCI hotplug).
Well, for PCI hotplug you will still need to execute a
XEN_DOMCTL_assign_device hypercall in order to assign the device, at
which point you could pass the vPCI flag.
I am probably missing something here. If you don't pass the vPCI flag at
domain creation, wouldn't it mean that hostbridge would not be created
until later? Are you thinking to make it unconditionally or hotplug it
(even that's even possible)?
What you likely want at domain create is whether the IOMMU should be
enabled or not, as we no longer allow late enabling of the IOMMU once
the domain has been created.
One question I have is whether Arm plans to allow exposing fully
emulated devices on the PCI config space, or that would be limited to
PCI device passthrough?
In the longer term, I would expect to have a mix of physical and
emulated device (e.g. virtio).
IOW: should an emulated PCI root complex be unconditionally exposed to
guests so that random ioreq servers can register for SBDF slots?
I would say no. The vPCI should only be added when the configuration
requested it. This is to avoid exposing unnecessary emulation to a
domain (not everyone will want to use a PCI hostbridge).
After that, the device assignment hypercall could have a way to say whether
the device will be emulated by vPCI. But I don't think this is necessary to
have from day one as the ABI will be not stable (this is a DOMCTL).
Indeed, it's not a stable interface, but we might as well get
something sane if we have to plumb it through the tools. Either if
it's a domain create flag or a device attach flag you will need some
plumbing to do at the toolstack level, at which point we might as well
use an interface that doesn't have arbitrary limits.
I think we need both flags. In your approach you seem to want to either
have the hostbridge created unconditionally or hotplug it (if that's
even possible).
However, I don't think we should have the vPCI unconditionally created
and we should still allow the toolstack to say at domain creation that
PCI will be used.
Cheers,
--
Julien Grall
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