Re: [PATCH v4 01/12] KVM: extend struct kvm_msi to hold a 32-bit device ID

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

On Wed, May 25, 2016 at 05:16:54PM +0100, Marc Zyngier wrote:
> On 25/05/16 16:55, Andre Przywara wrote:
> > Hi Christoffer,
> > 
> > On 03/04/16 10:15, Christoffer Dall wrote:
> >> On Sat, Mar 26, 2016 at 02:13:59AM +0000, 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 cb2ef0b..8f7351d 100644
> >>> --- a/Documentation/virtual/kvm/api.txt
> >>> +++ b/Documentation/virtual/kvm/api.txt
> >>> @@ -2163,10 +2163,18 @@ struct kvm_msi {
> >>>  	__u32 address_hi;
> >>>  	__u32 data;
> >>>  	__u32 flags;
> >>> -	__u8  pad[16];
> >>> +	__u32 devid;
> >>
> >> Are we imposing any unfortunate restrictions for other architectures by
> >> using a u32 over a u64 for the device ID?
> > 
> > Mmmh, good point. I guess not only for other architectures, but also for
> > the future in general.
> > 
> > Are there any objections against increasing this to a u64?
> 
> I'm not sure this is really necessary. A PCI RID is 16bit, and expanding
> it to 32bit is already making quite a bit of space for further extension.
> 
> Also, some architecture might be unhappy of having a 64bit quantity that
> is not 64bit aligned...
> 
ok, thanks for the explanation.

-Christoffer