Re: [Qemu-devel] [RFC] Next gen kvm api

[Avi Kivity]

On 02/07/2012 04:39 PM, Alexander Graf wrote:
> > 
> > Syscalls are orthogonal to that - they're to avoid the fget_light() and to tighten the vcpu/thread and vm/process relationship.
>
> How about keeping the ioctl interface but moving vcpu_run to a syscall then?

I dislike half-and-half interfaces even more.  And it's not like the
fget_light() is really painful - it's just that I see it occasionally in
perf top so it annoys me.

>  That should really be the only thing that belongs into the fast path, right? Every time we do a register sync in user space, we do something wrong. Instead, user space should either
>
>   a) have wrappers around register accesses, so it can directly ask for specific registers that it needs
> or
>   b) keep everything that would be requested by the register synchronization in shared memory

Always-synced shared memory is a liability, since newer hardware might
introduce on-chip caches for that state, making synchronization
expensive.  Or we may choose to keep some of the registers loaded, if we
have a way to trap on their use from userspace - for example we can
return to userspace with the guest fpu loaded, and trap if userspace
tries to use it.

Is an extra syscall for copying TLB entries to user space prohibitively
expensive?

> > 
> >> , keep the rest in user space.
> >> >
> >> >
> >> >  When a device is fully in the kernel, we have a good specification of the ABI: it just implements the spec, and the ABI provides the interface from the device to the rest of the world.  Partially accelerated devices means a much greater effort in specifying exactly what it does.  It's also vulnerable to changes in how the guest uses the device.
> >> 
> >> Why? For the HPET timer register for example, we could have a simple MMIO hook that says
> >> 
> >>   on_read:
> >>     return read_current_time() - shared_page.offset;
> >>   on_write:
> >>     handle_in_user_space();
> > 
> > It works for the really simple cases, yes, but if the guest wants to set up one-shot timers, it fails.  
>
> I don't understand. Why would anything fail here? 

It fails to provide a benefit, I didn't mean it causes guest failures.

You also have to make sure the kernel part and the user part use exactly
the same time bases.

> Once the logic that's implemented by the kernel accelerator doesn't fit anymore, unregister it.

Yeah.

>
> > Also look at the PIT which latches on read.
> > 
> >> 
> >> For IDE, it would be as simple as
> >> 
> >>   register_pio_hook_ptr_r(PIO_IDE, SIZE_BYTE,&s->cmd[0]);
> >>   for (i = 1; i<  7; i++) {
> >>     register_pio_hook_ptr_r(PIO_IDE + i, SIZE_BYTE,&s->cmd[i]);
> >>     register_pio_hook_ptr_w(PIO_IDE + i, SIZE_BYTE,&s->cmd[i]);
> >>   }
> >> 
> >> and we should have reduced overhead of IDE by quite a bit already. All the other 2k LOC in hw/ide/core.c don't matter for us really.
> > 
> > 
> > Just use virtio.
>
> Just use xenbus. Seriously, this is not an answer.

Why not?  We invested effort in making it as fast as possible, and in
writing the drivers.  IDE will never, ever, get anything close to virtio
performance, even if we put all of it in the kernel.

However, after these examples, I'm more open to partial acceleration
now.  I won't ever like it though.

> >> >
> >> >>    - VGA
> >> >>    - IDE
> >> >
> >> >  Why?  There are perfectly good replacements for these (qxl, virtio-blk, virtio-scsi).
> >> 
> >> Because not every guest supports them. Virtio-blk needs 3rd party drivers. AHCI needs 3rd party drivers on w2k3 and wxp. 

3rd party drivers are a way of life for Windows users; and the
incremental benefits of IDE acceleration are still far behind virtio.

> I'm pretty sure non-Linux non-Windows systems won't get QXL drivers. 

Cirrus or vesa should be okay for them, I don't see what we could do for
them in the kernel, or why.

> Same for virtio.
> >> 
> >> Please don't do the Xen mistake again of claiming that all we care about is Linux as a guest.
> > 
> > Rest easy, there's no chance of that.  But if a guest is important enough, virtio drivers will get written.  IDE has no chance in hell of approaching virtio-blk performance, no matter how much effort we put into it.
>
> Ever used VMware? They basically get virtio-blk performance out of ordinary IDE for linear workloads.

For linear loads, so should we, perhaps with greater cpu utliization.

If we DMA 64 kB at a time, then 128 MB/sec (to keep the numbers simple)
means 0.5 msec/transaction.  Spending 30 usec on some heavyweight exits
shouldn't matter.

> > 
> >> KVM's strength has always been its close resemblance to hardware.
> > 
> > This will remain.  But we can't optimize everything.
>
> That's my point. Let's optimize the hot paths and be good. As long as we default to IDE for disk, we should have that be fast, no?

We should make sure that we don't default to IDE.  Qemu has no knowledge
of the guest, so it can't default to virtio, but higher level tools can
and should.

> >> 
> >> Well, we don't always have shadow page tables. Having hints for unmapped guest memory like this is pretty tricky.
> >> We're currently running into issues with device assignment though, where we get a lot of small slots mapped to real hardware. I'm sure that will hit us on x86 sooner or later too.
> > 
> > For x86 that's not a problem, since once you map a page, it stays mapped (on modern hardware).
>
> Ah, because you're on NPT and you can have MMIO hints in the nested page table. Nifty. Yeah, we don't have that luxury :).

Well the real reason is we have an extra bit reported by page faults
that we can control.  Can't you set up a hashed pte that is configured
in a way that it will fault, no matter what type of access the guest
does, and see it in your page fault handler?

I'm guessing guest kernel ptes don't get evicted often.

> > 
> >> 
> >> >
> >> >>  That only works when then internal slot structure is hidden from user space though.
> >> >
> >> >  Why?
> >> 
> >> Because if user space thinks it's slots and in reality it's a tree that doesn't match. If you decouple the external view from the internal view, it works again.
> > 
> > Userspace needs to provide a function hva = f(gpa).  Why does it matter how the function is spelled out?  Slots happen to be a concise representation.  Transform the function all you like in the kernel, as long as you preserve all the mappings.
>
> I think we're talking about the same thing really.

So what's your objection to slots?

> >>   http://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg66155.html
> >> 
> > 
> > Yeah - s390 is always different.  On the current interface synchronous registers are easy, so why not.  But I wonder if it's really critical.
>
> It's certainly slick :). We do the same for the TLB on e500, just with a separate ioctl to set the sharing up.

It's also dangerous wrt future hardware, as noted above.

-- 
error compiling committee.c: too many arguments to function

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