Messages in this thread | | | Subject | Re: [PATCH] irqchip/gic-v3-its: Clear Valid before writing any bits else in VPENDBASER | From | Zenghui Yu <> | Date | Wed, 26 Feb 2020 09:35:26 +0800 |
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On 2020/2/26 3:45, Marc Zyngier wrote: > Hi Zenghui, > > On 2020-02-25 02:06, Zenghui Yu wrote: >> Hi Marc, >> >> On 2020/2/25 7:47, Marc Zyngier wrote: >>> Hi Zenghui, >>> >>> On 2020-02-24 02:50, Zenghui Yu wrote: >>>> The Valid bit must be cleared before changing anything else when >>>> writing >>>> GICR_VPENDBASER to avoid the UNPREDICTABLE behavior. This is exactly >>>> what >>>> we've done on 32bit arm, but not on arm64. >>> >>> I'm not quite sure how you decide that Valid must be cleared before >>> changing >>> anything else. The reason why we do it on 32bit is that we cannot update >>> the full 64bit register at once, so we start by clearing Valid so that >>> we can update the rest. arm64 doesn't require that. >> >> The problem came out from discussions with our GIC engineers and what we >> talked about at that time was IHI 0069E 9.11.36 - the description of the >> Valid field: >> >> "Writing a new value to any bit of GICR_VPENDBASER, other than >> GICR_VPENDBASER.Valid, when GICR_VPENDBASER.Valid==1 is UNPREDICTABLE." >> >> It looks like we should first clear the Valid and then write something >> else. We might have some mis-understanding about this statement.. > > So that's the v4.0 version of VPENDBASER. On v4.0, you start by clearing > Valid, not changing any other bit. Subsequent polling of the leads to > the PendingLast bit once Dirty clears. The current code follows this > principle. > >>> For the rest of discussion, let's ignore GICv4.1 32bit support (I'm >>> pretty sure nobody cares about that). >>> >>>> This works fine on GICv4 where we only clear Valid for a vPE >>>> deschedule. >>>> With the introduction of GICv4.1, we might also need to talk >>>> something else >>>> (e.g., PendingLast, Doorbell) to the redistributor when clearing the >>>> Valid. >>>> Let's port the 32bit gicr_write_vpendbaser() to arm64 so that >>>> hardware can >>>> do the right thing after descheduling the vPE. >>> >>> The spec says that: >>> >>> "For a write that writes GICR_VPENDBASER.Valid from 1 to 0, if >>> GICR_VPENDBASER.PendingLast is written as 1 then >>> GICR_VPENDBASER.PendingLast >>> takes an UNKNOWN value and GICR_VPENDBASER.Doorbell is treated as >>> being 0." >>> >>> and >>> >>> "When GICR_VPENDBASER.Valid is written from 1 to 0, if there are >>> outstanding >>> enabled pending interrupts GICR_VPENDBASER.Doorbell is treated as 0." >>> >>> which indicate that PendingLast/Doorbell have to be written at the >>> same time >>> as we clear Valid. >> >> Yes. I obviously missed these two points when writing this patch. >> >>> Can you point me to the bit of the v4.1 spec that makes >>> this "clear Valid before doing anything else" requirement explicit? >> >> No, nothing in v4.1 spec supports me :-( The above has been forwarded >> to Hisilicon and I will confirm these with them. It would be easy for >> hardware to handle the PendingLast/DB when clearing Valid, I think. > > v4.1 changes the way VPENDBASER works in a number of way. Clearing Valid > allows > a "handshake": At the point of making the vPE non-resident, to specify the > expected behaviour of the redistributor once the residency has been > completed. > This includes requesting the doorbell or telling the GIC that we don't > care to > know about PendingLast. > > This is effectively a relaxation of the v4.0 behaviour. I believe the > current > state of the driver matches both specs (not using common code though).
Yes, I agree with all of the above. Thanks for your confirmation and please ignore this patch.
Thanks, Zenghui
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