Re: [PATCH v8 03/17] KVM: extend struct kvm_msi to hold a 32-bit device ID

[Christoffer Dall <christoffer.dall@linaro.org>]

On Wed, Jul 06, 2016 at 10:54:58PM +0100, André Przywara wrote:
> On 06/07/16 22:06, Christoffer Dall wrote:
> > On Tue, Jul 05, 2016 at 12:22:55PM +0100, Andre Przywara wrote:
> >> The ARM GICv3 ITS MSI controller requires a device ID to be able to
> >> assign the proper interrupt vector. On real hardware, this ID is
> >> sampled from the bus. To be able to emulate an ITS controller, extend
> >> the KVM MSI interface to let userspace provide such a device ID. For
> >> PCI devices, the device ID is simply the 16-bit bus-device-function
> >> triplet, which should be easily available to the userland tool.
> >>
> >> Also there is a new KVM capability which advertises whether the
> >> current VM requires a device ID to be set along with the MSI data.
> >> This flag is still reported as not available everywhere, later we will
> >> enable it when ITS emulation is used.
> >>
> >> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> >> Reviewed-by: Eric Auger <eric.auger@linaro.org>
> >> ---
> >>  Documentation/virtual/kvm/api.txt | 12 ++++++++++--
> >>  include/uapi/linux/kvm.h          |  5 ++++-
> >>  2 files changed, 14 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> >> index 09efa9e..6551311 100644
> >> --- a/Documentation/virtual/kvm/api.txt
> >> +++ b/Documentation/virtual/kvm/api.txt
> >> @@ -2175,10 +2175,18 @@ struct kvm_msi {
> >>  	__u32 address_hi;
> >>  	__u32 data;
> >>  	__u32 flags;
> >> -	__u8  pad[16];
> >> +	__u32 devid;
> >> +	__u8  pad[12];
> >>  };
> >>  
> >> -No flags are defined so far. The corresponding field must be 0.
> >> +flags: KVM_MSI_VALID_DEVID: devid contains a valid value
> >> +devid: If KVM_MSI_VALID_DEVID is set, contains a unique device identifier
> >> +       for the device that wrote the MSI message.
> >> +       For PCI, this is usually a BFD identifier in the lower 16 bits.
> >> +
> >> +The per-VM KVM_CAP_MSI_DEVID capability advertises the need to provide
> >> +the device ID. If this capability is not set, userland cannot rely on
> >> +the kernel to allow the KVM_MSI_VALID_DEVID flag being set.
> > 
> > If KVM_CAP_MSI_DEVID is set, is it an error to provide a struct kvm_msi
> > without the KVM_MSI_VALID_DEVID flag set, or not necessarily?
> 
> In the moment we return an error when the bit is not set. But
> theoretically a guest could have other (in-kernel emulated) MSI
> controllers which don't require a device ID, though currently there is none.
> The reason for both the KVM capability and the flag on the ioctl is for
> mutual assurance that both userland and the kernel know about this ITS
> specific extension.
> This KVM_SIGNAL_MSI API is rather generic, so I don't want to
> unnecessarily slam the door on other emulated ITS controllers.
> 

That's a reasonable argument, that you could have multiple MSI
controllers with/without device ids.

Thanks,
-Christoffer