[Qemu-devel] Re: POLL: Why do you use kqemu?

[Jan Kiszka <jan.kiszka@web.de>]

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Blue Swirl wrote:
> On 6/7/09, Jan Kiszka <jan.kiszka@web.de> wrote:
>> Blue Swirl wrote:
>>  > On 6/7/09, Jan Kiszka <jan.kiszka@web.de> wrote:
>>  >> Avi Kivity wrote:
>>  >>  > Jan Kiszka wrote:
>>  >>  >>> Maybe the backwards compatibility features should be ported to QEMU?
>>  >>  >>> For example, is there a workaround for
>>  >>  >>> #error Missing KVM capability KVM_CAP_DESTROY_MEMORY_REGION_WORKS
>>  >>  >>> ?
>>  >>  >>>
>>  >>  >>
>>  >>  >> Given that we have always-up-to-date kvm-kmod packages with support down
>>  >>  >> to reasonable kernel versions, I would prefer to keep upstream clean
>>  >>  >> from old workarounds. They should only be needed for issues found very
>>  >>  >> recently (KVM_CAP_JOIN_MEMORY_REGIONS_WORKS) or that might be found in
>>  >>  >> the future.
>>  >>  >>
>>  >>  >
>>  >>  > Requiring the latest up-to-date modules is pushing the problem to the
>>  >>  > users.  Sometimes there is no choice, but when there is, the
>>  >>  > implementation that cares about its uses prefer unclean code and
>>  >>  > functionality over perfection and brokenness.
>>  >>
>>  >>
>>  >> Let's make it more concrete:
>>  >>
>>  >>  By the time upstream is as well tested, feature-rich and with comparable
>>  >>  performance as qemu-kvm, its current baseline requirement (2.6.29 due to
>>  >>  KVM_CAP_DESTROY_MEMORY_REGION_WORKS) will no longer be a problem to most
>>  >>  normal users. Until then they are better off with qemu-kvm anyway.
>>  >>
>>  >>  So all I wanted to express is that I see no point in merging workarounds
>>  >>  upstream that hardly anyone will need but that restrict non-kvm code in
>>  >>  upstream. Basically I have the current line along
>>  >>  KVM_CAP_DESTROY_MEMORY_REGION_WORKS / clean memory slot management in
>>  >>  mind. Anything older should be skipped when merging upstream. And unless
>>  >>  something more problematic comes along (rather unlikely), 2.6.29 or
>>  >>  compatible kvm-kmod is a reasonable minimum requirement for the long term.
>>  >
>>  > I pulled qemu-kvm and it looks to me that
>>  > KVM_CAP_DESTROY_MEMORY_REGION_WORKS and the derived functions
>>  > kvm_destroy_memory_region_works() and must_use_aliases_*() are only
>>  > used in very few places. Do I miss something, how can this be of any
>>  > restriction?
>>
>>
>> Check e.g the diff of hw/vga.c against upstream: All the magic dances
>>  there are required as qemu-kvm tracking cpu_register_physical_memory and
>>  kvm_log_start cannot cope with all the patterns normal qemu code comes
>>  up with. Upstream slot management now provides the same features
>>  (including migration) like qemu-kvm, it just does not deal with legacy,
>>  thus it does not have to patch qemu code (rather, we were able to remove
>>  some already merged hooks - vga_dirty_log_stop).
> 
> Still not very restrictive, all this could be encapsulated with for
> example macro COMPAT_NO_DMRW which could be removed when we don't care
> anymore. Next?

Really, it's not worth the maintenance pain: Every new device emulation
code that wants to be KVM-legacy-compatible would need to be written
like that. And unless you are familiar with the slot management
internals, the "correct" pattern will not be obvious.

I've just finished a patch for configure and for the runtime code to
tell the user what the maturer alternatives are. Will send it out in a
minute.

Jan


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