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Reply-To: Gavin Shan <gshan@redhat.com>
Subject: Re: [PATCH 1/4] KVM: arm64: Allow saving vgic3 LPI pending status in
 no running vcpu context
To:     Marc Zyngier <maz@kernel.org>
Cc:     Oliver Upton <oliver.upton@linux.dev>, kvmarm@lists.linux.dev,
        kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
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        catalin.marinas@arm.com, will@kernel.org, ricarkol@google.com,
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        shan.gavin@gmail.com
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From:   Gavin Shan <gshan@redhat.com>
Message-ID: <685ae61a-951f-e611-7491-948f19f1827e@redhat.com>
Date:   Fri, 20 Jan 2023 10:04:50 +1100
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Hi Marc,

On 1/20/23 2:47 AM, Marc Zyngier wrote:
> On Thu, 19 Jan 2023 01:11:44 +0000,
> Gavin Shan <gshan@redhat.com> wrote:
>>
>> I will have vgic_write_guest_lock() in v2. Note that those 3 paths can't be
>> running in parallel since one switch is shared by them. Alternatively, we
>> extend struct vgic_dist::save_tables_in_progress from 'bool' to 'unsigned long'.
>> Several bit is defined for each site as below. In this way, the 3 paths can be
>> running in parallel:
>>
>>    unsigned long struct vgic_dist::save_tables_in_progress
>>
>>    #define VGIC_DIST_SAVE_ITS_ITE		0	/* ITS Translation Entry */
>>    #define VGIC_DIST_SAVE_ITS_DTE		1	/* ITS Device Table Entry */
>>    #define VGIC_DIST_SAVE_ITS_CTE		2	/* ITS Collection Table Entry */
>>    #define VGIC_DIST_SAVE_ITS_CT			3	/* ITS Collection Table */
>>    #define VGIC_DIST_SAVE_VGIC3_LPI		4	/* VGIC3 LPI Pending Status */
>>    #define VGIC_DIST_SAVE_VGIC3_PENDING_TABLE	5	/* VGIC3 Pending Table */
>>
>> The drawback is the calls are limited to 64. If those 3 paths can't be running
>> in parallel, we needn't the extension at all.
> 
> It should all be completely sequential. KVM_DEV_ARM_ITS_SAVE_TABLES
> runs in a context where everything is locked, and so is
> VGIC_DIST_SAVE_VGIC3_PENDING_TABLE.
> 

Thanks for your confirm. Yeah, it's sequential because 'kvm->lock' is
hold on KVM_DEV_ARM_ITS_SAVE_TABLES and VGIC_DIST_SAVE_VGIC3_PENDING_TABLE.
So all good to have one shared switch. v2 will be posted pretty soon.

Thanks,
Gavin