Re: [PATCH v4 01/22] KVM: arm/arm64: Add vITS save/restore API documentation

Marc Zyngier
2017-04-08T17:29:11Z

On Sat, Apr 08 2017 at 02:15:52 PM, Christoffer Dall <christoffer.dall@linaro.org> wrote:
> On Sat, Apr 08, 2017 at 11:03:51AM +0100, Marc Zyngier wrote:
>> On Mon, Mar 27 2017 at 10:30:51 AM, Eric Auger <eric.auger@redhat.com> wrote:
>> > Add description for how to access vITS registers and how to flush/restore
>> > vITS tables into/from memory
>> >
>> > Signed-off-by: Eric Auger <eric.auger@redhat.com>
>> >
>> > ---
>> > v3 -> v4:
>> > - take into account Peter's comments:
>> > - typos
>> > - KVM_DEV_ARM_VGIC_GRP_ITS_TABLES kvm_device_attr = 0
>> > - add a validity bit in DTE
>> > - document all fields in CTE and ITE
>> > - document ABI revision
>> > - take into account Andre's comments:
>> > - document restrictions about GITS_CREADR writing and GITS_IIDR
>> > - document -EBUSY error if one or more VCPUS are runnning
>> > - document 64b registers only can be accessed with 64b access
>> > - itt_addr field matches bits [51:8] of the itt_addr
>> >
>> > v1 -> v2:
>> > - DTE and ITE now are 8 bytes
>> > - DTE and ITE now indexed by deviceid/eventid
>> > - use ITE name instead of ITTE
>> > - mentions ITT_addr matches bits [51:8] of the actual address
>> > - mentions LE layout
>> > ---
>> > Documentation/virtual/kvm/devices/arm-vgic-its.txt | 118 +++++++++++++++++++++
>> > 1 file changed, 118 insertions(+)
>> >
>> > diff --git a/Documentation/virtual/kvm/devices/arm-vgic-its.txt b/Documentation/virtual/kvm/devices/arm-vgic-its.txt
>> > index 6081a5b..0902d20 100644
>> > --- a/Documentation/virtual/kvm/devices/arm-vgic-its.txt
>> > +++ b/Documentation/virtual/kvm/devices/arm-vgic-its.txt
>> > @@ -36,3 +36,121 @@ Groups:
>> > -ENXIO: ITS not properly configured as required prior to setting
>> > this attribute
>> > -ENOMEM: Memory shortage when allocating ITS internal data
>> > +
>> > + KVM_DEV_ARM_VGIC_GRP_ITS_REGS
>> > + Attributes:
>> > + The attr field of kvm_device_attr encodes the offset of the
>> > + ITS register, relative to the ITS control frame base address
>> > + (ITS_base).
>> > +
>> > + kvm_device_attr.addr points to a __u64 value whatever the width
>> > + of the addressed register (32/64 bits). 64 bit registers can only
>> > + be accessed with full length.
>> > +
>> > + Writes to read-only registers are ignored by the kernel except for:
>> > + - GITS_READR. It needs to be restored otherwise commands in the queue
>> > + will be re-executed after CWRITER setting. Writing this register is
>> > + allowed if the ITS is not enabled (GITS_CTLR.enable = 0). Also it
>> > + needs to be restored after GITS_CBASER since a write to GITS_CBASER
>> > + resets GITS_CREADR.
>> > + - GITS_IIDR. Its Revision field encodes the table layout ABI revision.
>> > +
>> > + For other registers, getting or setting a register has the same
>> > + effect as reading/writing the register on real hardware.
>> > + Errors:
>> > + -ENXIO: Offset does not correspond to any supported register
>> > + -EFAULT: Invalid user pointer for attr->addr
>> > + -EINVAL: Offset is not 64-bit aligned
>> > + -EBUSY: one or more VCPUS are running
>> > +
>> > + KVM_DEV_ARM_VGIC_GRP_ITS_TABLES
>> > + Attributes
>> > + The attr field of kvm_device_attr must be zero.
>> > +
>> > + request the flush-save/restore of the ITS tables, namely
>> > + the device table, the collection table, all the ITT tables,
>> > + the LPI pending tables. On save, the tables are flushed
>> > + into guest memory at the location provisioned by the guest
>> > + in GITS_BASER (device and collection tables), in the MAPD
>> > + command (ITT_addr), GICR_PENDBASERs (pending tables).
>>
>> May I echo Andre's concern here? Seeing the GICR_PENDBASER registers
>> here feels wrong. The pending table is a redistributor concept, and is
>> local to it. Even more, it is perfectly possible to use the
>> redistributors (and its pending tables) without an ITS by using the
>> GICR_SETLPIR registers.
>>
>> Granted, we do not support this (and I'm *not* looking forward to
>> supporting it), but I would rather implement a redistributor flush than
>> using the ITS to indirectly force it. That's not how the HW works, and I
>> don't think we should deviate from it.
>>
>> I appreciate this brings additional complexity to userspace (having to
>> iterate over the vcpus or the redistributors is likely more costly than
>> hitting the ITS which is likely to be unique), but sticking to the
>> letter of the architecture is probably our best bet. Each time we tried
>> to deviate from it, we've had to backtrack.
>
> I agree. I think the possibility of using LPIs without an ITS is the
> winning argument.
>
> However, I could see it being reasonable to have a single call that
> userspace can make on the vgic device to ask it to flush all the
> redistributors at the same time. We defined the cpu sysreg interface
> via the vgic device as well, so I don't think we're breaking anything,
> and in fact, it may be good to have a single sync-point where the kernel
> can atomically say "all redistributors are flushed correctly" or
> "something bad happened". Do we have any arguments for wanting to be
> able flush redistributors independently?

Not as such. The redistributors may support implementation-specific ways
of ensuring synchronization with the PT (for power management purposes,
for example), but there is no architected way of ensuring it.

So I agree that having a single synchronization hook for all
redistributors in the system is probably the right thing to do. This is
simple enough for userspace, and we already have the required code in
this series -- we just need to tie it to an additional property on the
GICv3 group.

Eric: can you explore this and let us know if there is any issue we
haven't foreseen yet?

Thanks,

M.
--
Jazz is not dead. It just smells funny.