[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 15/33] docs: update ACPI CPU hotplug spec with n
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH 15/33] docs: update ACPI CPU hotplug spec with new protocol |
Date: |
Wed, 1 Jun 2016 00:09:57 +0300 |
On Tue, May 31, 2016 at 05:07:41PM +0200, Igor Mammedov wrote:
> On Tue, 31 May 2016 07:49:16 +0300
> "Michael S. Tsirkin" <address@hidden> wrote:
>
> > On Tue, May 17, 2016 at 04:43:07PM +0200, Igor Mammedov wrote:
> > > Signed-off-by: Igor Mammedov <address@hidden>
> > > ---
> > > docs/specs/acpi_cpu_hotplug.txt | 88
> > > +++++++++++++++++++++++++++++++++++------
> > > 1 file changed, 76 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/docs/specs/acpi_cpu_hotplug.txt
> > > b/docs/specs/acpi_cpu_hotplug.txt
> > > index 340b751..c5bce6a 100644
> > > --- a/docs/specs/acpi_cpu_hotplug.txt
> > > +++ b/docs/specs/acpi_cpu_hotplug.txt
> > > @@ -4,21 +4,85 @@ QEMU<->ACPI BIOS CPU hotplug interface
> > > QEMU supports CPU hotplug via ACPI. This document
> > > describes the interface between QEMU and the ACPI BIOS.
> > >
> > > -ACPI GPE block (IO ports 0xafe0-0xafe3, byte access):
> > > ------------------------------------------
> > > -
> > > -Generic ACPI GPE block. Bit 2 (GPE.2) used to notify CPU
> > > -hot-add/remove event to ACPI BIOS, via SCI interrupt.
> > > +ACPI BIOS GPE.2 handler is dedicated for notifying OS about CPU hot-add
> > > +and hot-remove events.
> > >
> > > +============================================
> > > +Legacy ACPI CPU hotplug interface registers:
> > > +--------------------------------------------
> > > CPU present bitmap for:
> > > + One bit per CPU. Bit position reflects corresponding CPU APIC ID.
> > > Read-only.
> > > ICH9-LPC (IO port 0x0cd8-0xcf7, 1-byte access)
> > > PIIX-PM (IO port 0xaf00-0xaf1f, 1-byte access)
> > > ---------------------------------------------------------------
> > > -One bit per CPU. Bit position reflects corresponding CPU APIC ID.
> > > -Read-only.
> > > +QEMU sets corresponding CPU bit on hot-add event and issues SCI
> > > +with GPE.2 event set. CPU present map read by ACPI BIOS GPE.2 handler
> > > +to notify OS about CPU hot-add events. CPU hot-remove isn't supported.
> > > +
> > > +=====================================
> > > +ACPI CPU hotplug interface registers:
> > > +-------------------------------------
> > > +Register block base address:
> > > + ICH9-LPC IO port 0x0cd8
> > > + PIIX-PM IO port 0xaf00
> >
> > OK but this means we either use legacy or new one,
> > bot both, which is problematic for people using old seabios
> > without acpi loading support and with -M pc.
> >
> > I don't say we must support them with >255 CPUs,
> > but I do say we should make an effort for simple
> > setups with <255 CPUs.
>
>
> it works with QEMU provided(shipped) BIOS,
> it works for migration case as legacy stays enables because of -M
> src_legacy_machine
>
> more that 255 cpus will break old BIOS in different
> ways /corrupt/hang/ depending on BIOS version.
>
> I'm not sure we should care about using old BIOS
> with new QEMU+new machine type though and allow
> expectations to be beyond what hw vendors usually set.
> It's the same as real hw does i.e. new hardware
> shipped with new BIOS.
> If user insist on old BIOS+new machine type he still has
> property knob to force legacy mode.
>
> on the fist glance, it's probably not that very hard
> to switch IO ports handling from legacy to new interface
> by sending from new cpu-hotplug AML a command to do so,
> my concerns here is:
> * +1 more state to migrate
> * probably issues with migration as target started with
> different IO layout
> * IO window freed after switching from legacy to new,
> will not be available to guest as it started with
> legacy window consumed by CPUS.CRS.
> * that legacy switching business is only PC specific
> means having a knob to turn it on so it won't pollute ARM
>
> all in all it's probably too much headache to make sure
> that improbable usecase would work, so after considering
> this idea I've dropped it and did it the way it's now.
Well I think it's worth the effort. I agree it's tricky
to implement but we do maintain compatibility for years.
Being orthogonal with bios version is very helpful for
a variety of reasons such as debugging.
> > > +Register block size:
> > > + ACPI_CPU_HOTPLUG_REG_LEN = 12
> > > +
> > > +read access:
> >
> > So this implies acpi must scan all cpus on each event, and
> > this seems too aggressive.
> > I think we need something hierarchical where
> > you read one level and know which cpus to probe.
> That's what we do for mem-hotplug as it's not
> performance critical path.
>
> In addition to that depending on guest OS/version
> it will anyway do enumeration of all CPUs after
> our hotplug AML method scanned all CPUs and
> sent notifies.
>
> not that I'm in favor of complicating this protocol,
> but I wouldn't do it hierarchical,
> that's what Notify(BUS_CHECK) is supposed to do
> but it's broken on some guests.
Interesting. Which ones? Would they be easy to detect?
> So if I'd do a more complicated protocol,
> I'd do polling from AML side telling QEMU
> if (cpu = give_me_next_cpu_with_event())
> switch_to_cpu(cpu)
> ...
>
> that means adding extra state to migrate,
> probably ther might be other issues I'm not aware of yet.
>
> On the other hand mem-hotplug like interface
> so far hasn't caused any issues
> (that of cause doesn't mean that there aren't any).
>
> > > + offset:
> > > + [0x0-0x3] reserved
> > > + [0x4] CPU device status fields: (1 byte access)
> > > + bits:
> > > + 0: Device is enabled and may be used by guest
> > > + 1: Device insert event, used to distinguish device for which
> > > + no device check event to OSPM was issued.
> > > + It's valid only when bit 0 is set.
> > > + 2: Device remove event, used to distinguish device for which
> > > + no device eject request to OSPM was issued.
> > > + 3-7: reserved and should be ignored by OSPM
> > > + [0x5-0x7] reserved
> > > + [0x8] Command data: (DWORD access)
> > > + in case of error or unsupported command reads is 0xFFFFFFFF
> > > + current 'Command field' value:
> > > + 0: returns PXM value corresponding to device
> > > +
> > > +write access:
> > > + offset:
> > > + [0x0-0x3] CPU selector: (DWORD access)
> > > + selects active CPU device. All following accesses to other
> > > + registers will read/store data from/to selected CPU.
> >
> > I've been thinking - is it time to add an EmbeddedControl interface?
> > This way we have another namespace.
>
> There were patches on list with it some time ago
>
> "[PATCH RFC v2 0/7] coordinate cpu hotplug/unplug bewteen QEMU and kernel
> by EC"
>
> but we dropped it as bloatware because there weren't any
> real need for it as current IO port interfaces aren't
> going away any time soon and work just fine without EC.
Right. But now you are removing them for >255 CPUs
anyway.
> And if we were to move current interfaces into EC namespace
> we would have to do it only for new machines types
> and keep old code in place as well and a bunch of compat
> code for a company.
Hmm. Exactly like this current one?
> > Not insisting on it, just an idea.
> I wouldn't make it a necessary dependency and block this series.
> Might be worth to look at again when there is usecases for it
> without legacy stuff attached.
and then we'll have to maintain 3 interfaces?
> >
> > > + [0x4] CPU device control fields: (1 byte access)
> > > + bits:
> > > + 0: reserved, OSPM must clear it before writing to register.
> > > + 1: if set to 1 clears device insert event, set by OSPM
> > > + after it has emitted device check event for the
> > > + selected CPU device
> > > + 2: if set to 1 clears device remove event, set by OSPM
> > > + after it has emitted device eject request for the
> > > + selected CPU device
> > > + 3: if set to 1 initiates device eject, set by OSPM when it
> > > + triggers CPU device removal and calls _EJ0 method
> > > + 4-7: reserved, OSPM must clear them before writing to
> > > register
> > > + [0x5] Command field: (1 byte access)
> > > + value:
> > > + 0: following reads from 'Command data' register returns PXM
> > > + value of device
> > > + 1: following writes to 'Command data' register set OST event
> > > + register in QEMU
> > > + 2: following writes to 'Command data' register set OST status
> > > + register in QEMU
> > > + other values: reserved
> > > + [0x6-0x7] reserved
> > > + [0x8] Command data: (DWORD access)
> > > + current 'Command field' value:
> > > + 1: stores value into OST event register
> > > + 2: stores value into OST status register, triggers
> > > + ACPI_DEVICE_OST QMP event from QEMU to external
> > > applications
> > > + with current values of OST event and status registers.
> > > + other values: reserved
> > >
> > > -CPU hot-add/remove notification:
> > > ------------------------------------------------------
> > > -QEMU sets/clears corresponding CPU bit on hot-add/remove event.
> > > -CPU present map read by ACPI BIOS GPE.2 handler to notify OS of CPU
> > > -hot-(un)plug events.
> > > +Selecting CPU device beyond possible range has no effect on platform:
> > > + - write accesses to CPU hot-plug registers not documented above are
> > > + ignored
> > > + - read accesses to CPU hot-plug registers not documented above return
> > > + all bits set to 1.
> >
> > why not 0?
> isn't hardware usually return 1s on unhandled IO reads?
> (I even thought that's it was you who told me it)
PCI does but it's special.
>
> that's what has been done for memory hotplug, so I'm doing the same here.
>
> >
> > > --
> > > 1.8.3.1
> >
- [Qemu-devel] [PATCH v2 13/33] acpi: extend ACPI interface to provide send_event hook, (continued)
- [Qemu-devel] [PATCH 17/33] pc: add generic CPU unplug callbacks, Igor Mammedov, 2016/05/17
- [Qemu-devel] [PATCH 14/33] pc: use AcpiDeviceIfClass.send_event to issue GPE events, Igor Mammedov, 2016/05/17
- [Qemu-devel] [PATCH 18/33] pc: add 2.7 machine, Igor Mammedov, 2016/05/17
- [Qemu-devel] [PATCH 21/33] pc: piix4: initialize new CPU hotplug hw, Igor Mammedov, 2016/05/17
- [Qemu-devel] [PATCH 24/33] acpi: add CPU hotplug methods to DSDT, Igor Mammedov, 2016/05/17