linux-kernel - Re: [PATCH v5 19/22] KVM: arm64: Support SDEI ioctl commands on vCPU

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Date:   Fri, 25 Mar 2022 18:23:14 +0800
From:   Gavin Shan <gshan@...hat.com>
To:     Oliver Upton <oupton@...gle.com>
Cc:     kvmarm@...ts.cs.columbia.edu, maz@...nel.org,
        linux-kernel@...r.kernel.org, eauger@...hat.com,
        shan.gavin@...il.com, Jonathan.Cameron@...wei.com,
        pbonzini@...hat.com, vkuznets@...hat.com, will@...nel.org
Subject: Re: [PATCH v5 19/22] KVM: arm64: Support SDEI ioctl commands on vCPU

Hi Oliver,

On 3/25/22 4:37 PM, Oliver Upton wrote:
> On Fri, Mar 25, 2022 at 03:59:50PM +0800, Gavin Shan wrote:
>> On 3/24/22 1:55 AM, Oliver Upton wrote:
>>> On Tue, Mar 22, 2022 at 04:07:07PM +0800, Gavin Shan wrote:
>>>> This supports ioctl commands on vCPU to manage the various object.
>>>> It's primarily used by VMM to accomplish migration. The ioctl
>>>> commands introduced by this are highlighted as below:
>>>>
>>>>      * KVM_SDEI_CMD_GET_VCPU_EVENT_COUNT
>>>>        Return the total count of vCPU events, which have been queued
>>>>        on the target vCPU.
>>>>
>>>>      * KVM_SDEI_CMD_GET_VCPU_EVENT
>>>>      * KVM_SDEI_CMD_SET_VCPU_EVENT
>>>>        Get or set vCPU events.
>>>>
>>>>      * KVM_SDEI_CMD_GET_VCPU_STATE
>>>>      * KVM_SDEI_CMD_SET_VCPU_STATE
>>>>        Get or set vCPU state.
>>>
>>> All of this GET/SET stuff can probably be added to KVM_{GET,SET}_ONE_REG
>>> immediately. Just introduce new registers any time a new event comes
>>> along. The only event we have at the end of this series is the
>>> software-signaled event, with async PF coming later right?
>>>
>>> Some special consideration is likely necessary to avoid adding a
>>> register for every u64 chunk of data. I don't think we need to afford
>>> userspace any illusion of granularity with these, and can probably lump
>>> it all under one giant pseudoregister.
>>>
>>
>> Yes, KVM_{GET,SET}_ONE_REG is the ideal interface for migration. You're
>> correct we're only concerned by software signaled event and the one for
>> Async PF.
>>
>> I didn't look into Raghavendra's series deeply. Actually, a lump of
>> registers can be avoid after 2048 byte size is specified in its
>> encoding. I think 2048 bytes are enough for now since there are
>> only two supported events.
> 
> When I had said 'one giant pseudoregister' I actually meant 'one
> pseudoregister per event', not all of SDEI into a single
> structure. Since most of this is in flux now, it is hard to point
> out what/how we should migrate from conversation alone.
> 
> And on the topic of Raghavendra's series, I do not believe it is
> required anymore here w/ the removal of shared events, which I'm
> strongly in favor of.
> 
> Let's delve deeper into migration on the next series :)
> 

Ok, Thanks for your clarification about 'one giant pseudoregister'.
Lets have more discussion about the migration on next revision.
To be more clear, I plan to implement the base functionality,
where only the private event is supported. After it reaches into
mergeable or merged, we can post the add-on series to support
migration.

>> In the future, we probably have varied number of SDEI events to
>> be migrated. In that case, we need to add a new bit to the encoding
>> of the pseudo system register, so that VMM (QEMU) can support
>> variable sized system register and keep reading and writing on
>> these registers on migration:
>>
>>      PSEUDO_SDEI_ADDR: 64-bits in width
>>      PSEUDO_SDEI_DATA: has varied size
>>
>> PSEUDO_SDEI_ADDR is used to (1) Indicate the size of PSEUDO_SDEI_DATA
>> (2) The information to be read/written, for example the (shared/private)
>> registered events on VM and vCPU, VCPU state.
>>
>> PSEUDO_SDEI_DATA is used to (1) Retrieved information or that to be
>> written. (2) Flags to indicate current block of information is the
>> last one or not.
> 
> I don't think we have sufficient encoding space in the register ID
> to allow for arbitrary length registers. Any new registers for SDEI will
> need to fit into one of the predefined sizes. Note that we've already
> conditioned userspace to handle registers this way and anything else is
> an ABI change.
> 

Ok, I think we need padding to structures of the event, to make the
event object aligned to 64-bytes aligned, and VCPU state to 512-bytes
aligned. 64-bytes and 128-bytes registers have been supported.

Thanks,
Gavin