Re: [Qemu-devel] [RFC PATCH 16/30] q35/xen: Add Xen platform device support for Q35

[Alexey G <x1917x@gmail.com>]

On Mon, 12 Mar 2018 18:44:02 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

>On Tue, Mar 13, 2018 at 06:56:37AM +1000, Alexey G wrote:
>> On Mon, 12 Mar 2018 16:44:06 -0300
>> Eduardo Habkost <ehabkost@redhat.com> wrote:
>>   
>> >On Tue, Mar 13, 2018 at 04:34:01AM +1000, Alexey Gerasimenko
>> >wrote:  
>> >> Current Xen/QEMU method to control Xen Platform device on i440 is
>> >> a bit odd -- enabling/disabling Xen platform device actually
>> >> modifies the QEMU emulated machine type, namely xenfv <--> pc.
>> >> 
>> >> In order to avoid multiplying machine types, use a new way to
>> >> control Xen Platform device for QEMU -- "xen-platform-dev" machine
>> >> property (bool). To maintain backward compatibility with existing
>> >> Xen/QEMU setups, this is only applicable to q35 machine currently.
>> >> i440 emulation still uses the old method (i.e. xenfv/pc machine
>> >> selection) to control Xen Platform device, this may be changed
>> >> later to xen-platform-dev property as well.
>> >> 
>> >> This way we can use a single machine type (q35) and change just
>> >> xen-platform-dev value to on/off to control Xen platform device.
>> >> 
>> >> Signed-off-by: Alexey Gerasimenko <x1917x@gmail.com>
>> >> ---    
>> >[...]  
>> >> diff --git a/qemu-options.hx b/qemu-options.hx
>> >> index 6585058c6c..cee0b92028 100644
>> >> --- a/qemu-options.hx
>> >> +++ b/qemu-options.hx
>> >> @@ -38,6 +38,7 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
>> >>      "                dump-guest-core=on|off include guest memory
>> >> in a core dump (default=on)\n" "                mem-merge=on|off
>> >> controls memory merge support (default: on)\n" "
>> >> igd-passthru=on|off controls IGD GFX passthrough support
>> >> (default=off)\n"
>> >> +    "                xen-platform-dev=on|off controls Xen
>> >> Platform device (default=off)\n" "
>> >> aes-key-wrap=on|off controls support for AES key wrapping
>> >> (default=on)\n" "                dea-key-wrap=on|off controls
>> >> support for DEA key wrapping (default=on)\n" "
>> >> suppress-vmdesc=on|off disables self-describing migration
>> >> (default=off)\n"    
>> >
>> >What are the obstacles preventing "-device xen-platform" from
>> >working?  It would be better than adding a new boolean option to
>> >-machine.  
>> 
>> I guess the initial assumption was that changing the
>> xen_platform_device value in Xen's options may cause some additional
>> changes in platform configuration besides adding (or not) the Xen
>> Platform device, hence a completely different machine type was chosen
>> (xenfv).
>> 
>> At the moment pc,accel=xen/xenfv selection mostly governs
>> only the Xen Platform device presence. Also setting max_cpus to
>> HVM_MAX_VCPUS depends on it, but this doesn't applicable to a
>> 'pc,accel=xen' machine for some reason.
>> 
>> If applying HVM_MAX_VCPUS to max_cpus is really necessary I think
>> it's better to set it unconditionally for all 'accel=xen' HVM machine
>> types inside xen_enabled() block. Right now it's missing for
>> pc,accel=xen and q35,accel=xen.  
>
>If you are talking about MachineClass::max_cpus, note that it is
>returned by query-machines, so it's supposed to be a static
>value.  Changing it a runtime would mean the query-machines value
>is incorrect.
>
>Is HVM_MAX_CPUS higher or lower than 255?  If it's higher, does
>it mean the current value on pc and q35 isn't accurate?

HVM_MAX_VCPUS is 128 currently, but there is an ongoing work from Intel
to support more vcpus and >8bit APIC IDs, so this number will likely
change soon.

According to the code, using HVM_MAX_VCPUS in QEMU is a bit excessive as
the maximum number of vcpus is controlled on Xen side anyway. Currently
HVM_MAX_VCPUS is used in a one-time check for the maxcpus value (which
itself comes from libxl).
I think for future compatibility it's better to set mc->max_cpus to
HVM_MAX_VCPUS for all accel=xen HVM-supported machine types, not just
xenfv.

The '-device' approach you suggested seems more preferable than a
machine bool property, I'll try switching to it.

>Is HVM_MAX_CPUS something that needs to be enabled because of
>accel=xen or because or the xen-platform device?
>
>If it's just because of accel=xen, we could introduce a
>AccelClass::max_cpus() method (we also have KVM-imposed CPU count
>limits, currently implemented inside kvm_init()).