[virtio-dev] Re: [virtio-comment] Re: [PATCH 00/11] Introduce transitional mmr pci device

Michael S. Tsirkin
2023-04-07T09:35:25Z

On Mon, Apr 03, 2023 at 06:00:13PM -0400, Parav Pandit wrote:
>
>
> On 4/3/2023 5:04 PM, Michael S. Tsirkin wrote:
> > On Mon, Apr 03, 2023 at 08:25:02PM +0000, Parav Pandit wrote:
> > >
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Monday, April 3, 2023 2:02 PM
> > >
> > > > > Because vqs involve DMA operations.
> > > > > It is left to the device implementation to do it, but a generic wisdom
> > > > > is not implement such slow work in the data path engines.
> > > > > So such register access vqs can/may be through firmware.
> > > > > Hence it can involve a lot higher latency.
> > > >
> > > > Then that wisdom is wrong? tens of microseconds is not workable even for
> > > > ethtool operations, you are killing boot time.
> > > >
> > > Huh.
> > > What ethtool latencies have you experienced? Number?
> >
> > I know an order of tens of eth calls happens during boot.
> > If as you said each takes tens of ms then we are talking close to a second.
> > That is measureable.
> I said it can take, doesn't have to be always same for all the commands.
> Better to work with real numbers. :)
>
> Let me take an example to walk through.
>
> If a cvq or aq command takes 0.5msec, total of 100 such commands will take
> 50msec.
>
> Once a while if two of commands say take 5msec, will result in 50 -> 60
> msec.

Not too bad. then it seems it should not be a problem to tunnel config
over AQ then?

>
> > OK then. Then if it is a dead end then it looks weird to add a whole new
> > config space as memory mapped.
> >
> I am aligned with you to not add any new register as memory mapped for 1.x.
> Or access through device own's tvq is fine if such q can be initialized
> before during device reset (init) phase.
>
> I explained that legacy registers are sub-set of existing 1.x.
> They should not consume extra memory.
>
> Lets walk through the merits and negatives of both to conclude.
>
> > > > Let me try again.
>
> > If hardware vendors do not want to bear the costs of registers then they
> > will not implement devices with registers, and then the whole thing will
> > become yet another legacy thing we need to support. If legacy emulation
> > without IO is useful, then can we not find a way to do it that will
> > survive the test of time?
> legacy_register_transport_vq for VF can be a option, but not for PF
> emulation.

OK. Do we really care? Are you guys selling lots of high end cards
without SRIOV that it matters?

> More below.
>
> >
> > > Again, I want to emphasize that register read/write over tvq has merits with trade-off.
> > > And so the mmr has merits with trade-off too.
> > >
> > > Better to list them and proceed forward.
> > >
> > > Method-1: VF's register read/write via PF based transport VQ
> > > Pros:
> > > a. Light weight registers implementation in device for new memory region window
> >
> > Is that all? I mentioned more.
> >
> b. device reset is more optimal with transport VQ
> c. a hypervisor may want to check (but not necessary) register content
> d. Some unknown guest VM driver which modifies mac address and still expect
> atomicity can benefit if hypervisor wants to do extra checks

It's not hard to be more specific.
Old Linux kernels are like this, this was fixed with:

commit 7e58d5aea8abb993983a3f3088fd4a3f06180a1c
Author: Amos Kong <akong@redhat.com>
Date: Mon Jan 21 01:17:23 2013 +0000

Currently we write MAC address to pci config space byte by byte,
this means that we have an intermediate step where mac is wrong.
This patch introduced a new control command to set MAC address,
it's atomic.

about 10 years ago.

> > > Cons:
> > > a. Higher DMA read/write latency
> > > b. Device requires synchronization between non legacy memory mapped registers and legacy regs access via tvq
> >
> > Same as a separate mmemory bar really. Just don't do it. Either access
> > legacy or non legacy.
> >
> It is really not same to treat them equally as tvq encapsulation is
> different, and hw wouldn't prefer to treat them equally like regular memory
> writes.

I think yoiu missunderstand what I said. You listed a problem:
the same device can be accessed through both
a modern and a legacy interface.
I said that it is not a problem at all, there is no reason
to use both.

> Transitional device exposed by hypervisor contains both legacy I/O bar and
> also the memory mapped registers. So a guest vm can access both.

But it must not, and some devices break if you do.

> > > c. Can only work with the VF. Cannot work for thin hypervisor, which can map transitional PF to bare metal OS
> > > (also listed in cover letter)
> >
> > Is that a significant limitation? Why?
> It is a functional limitation for the PF, as PF has no parent.
> and PF can also utilize memory BAR.

Yes it's a limitation, I just don't see why we care.

> >
> > > Method-2: VF's register read/write via MMR (current proposal)
> > > Pros:
> > > a. Device utilizes the same legacy and non-legacy registers.
> >
> > > b. an order of magnitude lower latency due to avoidance of DMA on register accesses
> > > (Important but not critical)
> >
> > And no cons? Even if you could not see them yourself did I fail to express myself to such
> > an extent?
> >
> Method-1 pros covered the advantage of it over method-2, but yes worth to
> list here for completeness.
>
> Cons:
> requires creating new memory region window in the device for configuration
> access

Parav please take a look at the discussion so far as collect more cons
that were mentioned for the proposal, I definitely listed some and I
don't really want to repeat myself. I expect a proposal to be balanced,
not a sales pitch.

> > > > > No. Interrupt latency is in usec range.
> > > > > The major latency contributors in msec range can arise from the device side.
> > > >
> > > > So you are saying there are devices out there already with this MMR hack
> > > > baked in, and in hardware not firmware, so it works reasonably?
> > > It is better to not assert a solution a "hack",
> >
> > Sorry if that sounded offensive. a hack is not necessary a bad thing.
> > It's a quick solution to a very local problem, though.
> >
> It is a solution because device can do at near to zero extra memory for
> existing registers.
> Anyways, we have better technical details to resolve. :)
> Lets focus on it.
>
> > Yes motivation is one of the things I'm trying to work out here.
> > It does however not help that it's an 11 patch strong patchset
> > adding 500 lines of text for what is supposedly a small change.
> >
> Many of the patches are rework and incorrect to attribute to the specific
> feature.
>
> Like others it could have been one giant patch... but we see value in
> smaller patches..
>
> Using tvq is even bigger change than this.

The main thing is that there's no new ID so the PF device itself will
stay usable with existing drivers.

> So we shouldn't be afraid of
> making transitional device actually work using it with larger spec patch.
>
> > > Regarding tvq, I have some idea on how to improve the register read/writes so that its optimal for devices to implement.
> >
> > Sounds useful, and maybe if tvq addresses legacy need then focus on
> > that?
> >
>
> tvq specific for legacy register access make sense.
> Some generic tvq is abstract and dont see any relation here.
>
> So better to name it as legacy_reg_transport_vq (lrt_vq).

Again this assumes tvq will be rewritten on top of AQ.
I guess legacy can then become a new type of AQ command?

And maybe you want a memory mapped register for AQ commands? I know
Jason really wanted that.

> How about having below format?
>
> /* Format of 16B descriptors for lrt_vq
> * lrt_vq = legacy register tranport vq.
> */
> struct legacy_reg_req_vf {
> union {
> struct {
> le32 reg_wr_data;
> le32 reserved;
> } write;
> struct {
> le64 reg_read_addr;
> };
> };
> le8 rd_wr : 1; /* rd=0, wr=1 */
> le8 reg_byte_offset : 7;
> le8 req_tag; /* unique request tag on this vq */
> le16 vf_num;
>
> le16 flags; /* new flag below */
> le16 next;
> };
>
> #define VIRTQ_DESC_F_Q_DEFINED 8
> /* Content of the VQ descriptor other than flags field is VQ
> * specific and defined by the VQ type.
> */

Any way to allow accesses of arbitrary length?

--
MST