linux-kernel - Re: [RFC PATCH v1 1/4] irqchip/gic-v4.1: Plumb get_irqchip

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Message-ID: <316fe41d-f004-f004-4f31-6fe6e7ff64b7@huawei.com>
Date:   Tue, 1 Dec 2020 17:38:12 +0800
From:   luojiaxing <luojiaxing@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     Shenming Lu <lushenming@...wei.com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Eric Auger <eric.auger@...hat.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Christoffer Dall <christoffer.dall@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Cornelia Huck <cohuck@...hat.com>, "Neo Jia" <cjia@...dia.com>,
        <wanghaibin.wang@...wei.com>, <yuzenghui@...wei.com>
Subject: Re: [RFC PATCH v1 1/4] irqchip/gic-v4.1: Plumb get_irqchip_state VLPI
 callback


On 2020/11/28 18:18, Marc Zyngier wrote:
> On Sat, 28 Nov 2020 07:19:48 +0000,
> luojiaxing <luojiaxing@...wei.com> wrote:
>> Hi, shenming
>>
>>
>> I got few questions about this patch.
>>
>> Although it's a bit late and not very appropriate, I'd like to ask
>> before you send next version.
>>
>> On 2020/11/23 14:54, Shenming Lu wrote:
>>> From: Zenghui Yu <yuzenghui@...wei.com>
>>>
>>> Up to now, the irq_get_irqchip_state() callback of its_irq_chip
>>> leaves unimplemented since there is no architectural way to get
>>> the VLPI's pending state before GICv4.1. Yeah, there has one in
>>> v4.1 for VLPIs.
>>
>> I checked the invoking scenario of irq_get_irqchip_state and found no
>> scenario related to vLPI.
>>
>> For example, synchronize_irq(), it pass IRQCHIP_STATE_ACTIVE to which,
>> so in your patch, you will directly return and other is for vSGI,
>> GICD_ISPENDR, GICD_ICPENDR and so on.
> You do realise that LPIs have no active state, right?


yes, I know


> And that LPIs
> have a radically different programming interface to the rest of the GIC?


I found out that my mailbox software filtered out the other two patches, 
which led me to look at the patch alone, so it was weird. I already got 
the answer now.


>> The only one I am not sure is vgic_get_phys_line_level(), is it your
>> purpose to fill this callback, or some scenarios I don't know about
>> that use this callback.
> LPIs only offer edge signalling, so the concept of "line level" means
> absolutely nothing.
>
>>
>>> With GICv4.1, after unmapping the vPE, which cleans and invalidates
>>> any caching of the VPT, we can get the VLPI's pending state by
>>> peeking at the VPT. So we implement the irq_get_irqchip_state()
>>> callback of its_irq_chip to do it.
>>>
>>> Signed-off-by: Zenghui Yu <yuzenghui@...wei.com>
>>> Signed-off-by: Shenming Lu <lushenming@...wei.com>
>>> ---
>>>    drivers/irqchip/irq-gic-v3-its.c | 38 ++++++++++++++++++++++++++++++++
>>>    1 file changed, 38 insertions(+)
>>>
>>> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
>>> index 0fec31931e11..287003cacac7 100644
>>> --- a/drivers/irqchip/irq-gic-v3-its.c
>>> +++ b/drivers/irqchip/irq-gic-v3-its.c
>>> @@ -1695,6 +1695,43 @@ static void its_irq_compose_msi_msg(struct irq_data *d, struct msi_msg *msg)
>>>    	iommu_dma_compose_msi_msg(irq_data_get_msi_desc(d), msg);
>>>    }
>>>    +static bool its_peek_vpt(struct its_vpe *vpe, irq_hw_number_t
>>> hwirq)
>>> +{
>>> +	int mask = hwirq % BITS_PER_BYTE;
>>> +	void *va;
>>> +	u8 *pt;
>>> +
>>> +	va = page_address(vpe->vpt_page);
>>> +	pt = va + hwirq / BITS_PER_BYTE;
>>> +
>>> +	return !!(*pt & (1U << mask));
>>
>> How can you confirm that the interrupt pending status is the latest?
>> Is it possible that the interrupt pending status is still cached in
>> the GICR but not synchronized to the memory.
> That's a consequence of the vPE having been unmapped:
>
> "A VMAPP with {V,Alloc}=={0,1} cleans and invalidates any caching of
> the Virtual Pending Table and Virtual Configuration Table associated
> with the vPEID held in the GIC."


Yes, in addition to that, if a vPE is scheduled out of the PE, the cache 
clearing and write-back to VPT are also performed, I think.


However, I feel a litter confusing to read this comment at first ,  
because it is not only VMAPP that causes cache clearing.

I don't know why VMAPP was mentioned here until I check the other two 
patches ("KVM: arm64: GICv4.1: Try to save hw pending state in 
save_pending_tables").


So I think may be it's better to add some background description here.


Thanks

Jiaxing


>
> An implementation that wouldn't follow this simple rule would simply
> be totally broken, and unsupported.
>
> 	M.
>