Re: [Qemu-devel] [PATCH V3 2/2] arm_gicv3_kvm: kvm_dist_get/put: skip the registers banked by GICR

[Shannon Zhao <zhaoshenglong@huawei.com>]

On 2018/5/24 22:56, Peter Maydell wrote:
> On 24 May 2018 at 15:40, Auger Eric <eric.auger@redhat.com> wrote:
>> > Hi Peter,
>> >
>> > On 05/24/2018 04:16 PM, Peter Maydell wrote:
>>> >> Only for KVM, not for TCG, and it's the other way round: we
>>> >> end up with two lots of PPI/SGI space in the data structure
>>> >> by mistake. Let me fish out the comment I made on the v2 of this
>>> >> series:
>>> >>
>>> >> In the code in master, we have QEMU data structures
>>> >> (bitmaps, etc) which have one entry for each of GICV3_MAXIRQ
>>> >> irqs. That includes the RAZ/WI unused space for the SPIs/PPIs, so
>>> >> for a 1-bit-per-irq bitmap:
>>> >>  [0x00000000, irq 32, irq 33, .... ]
>>> >>
>>> >> When we fill in the values from KVM into these data structures,
>>> >> we start after the unused space, because the for_each_dist_irq_reg()
>>> >> macro starts with _irq = GIC_INTERNAL. But we forgot to adjust
>>> >> the offset value we use for the KVM access, so we start by
>>> >> reading the RAZ/WI values from KVM, and the data structure
>>> >> contents end up with:
>>> >>  [0x00000000, 0x00000000, irq 32, irq 33, ... ]
>>> >> (and the last irqs wouldn't get transferred).
>> > In kvm_dist_get_priority (new code), the offset is where we read and
>> > field is where we write, correct? Offset was shifted so we effectively
>> > read in KVM regs the num_irq-32 SPI states now but don't we start
>> > writing at the beginning of bmp, (ie s->gicd_ipriority), at PPI/SGI
>> > offset? What am I missing?
> Oops, yes, you're right. My explanation applies to the
> various other bitmaps, where we are accessing the
> fields in the data structure using gic_bmp_ptr32(bmp, irq),
> but not to gicd_ipriority[], which we are directly accessing
> starting with the first word, not by indexing via bmp[irq].
> 
> So we need to handle these two cases differently.
> You're correct that for gicd_ipriority[], the code in
> master reads and writes to that data structure as:
>  [0, 0, ..., 0, irq 32, irq 33, ..., 0, 0, ... 0]
> so all the values are in the right place but we:
>  (a) unnecessarily read/write zeroes for the PPI/SGI fields
>  (b) fail to transfer the last 32 interrupts
> 
> We can fix the gicd_ipriority[] case simply by adding
>    bmp = GIC_INTERNAL;
> before the assignment to 'field' in both kvm_dist_get_priority()
> and kvm_dist_put_priority(). This doesn't affect migration
> compatibility. We should do this separately from fixing the
> other bitmaps, because it's simpler.
> 
If we do bmp += GIC_INTERNAL, we should also add this to offset,
otherwise we will put the SGI/PPIs data to SPIs, right?

Thanks,
-- 
Shannon