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Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux

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El 14/09/15 a les 16.54, Stefano Stabellini ha escrit:
> On Mon, 14 Sep 2015, Roger Pau Monné wrote:
>> IMHO this splitting is just a workaround for the fact that we don't have
>> a 64KB PV block protocol, and this is the real problem that should be
>> solved.
> 
> 64K is a pure one guest kernel configuration option, not a platform wide
> option. The hypervisor interfaces are still the same, the ABI is the
> same and all the other guests are still the same, the Xen binary is
> still the same.

Yes, I understand that, but the PV block protocol is missing 64KB page
support, and that's a fact that cannot be ignored. To put an example, is
there a hardware SATA controller on ARM that doesn't support 64KB pages
and needs a similar workaround?

> A 64K block protocol could be a good performance imprevement, but should
> not be required to run kernels which have different config options.
> 
>> In the long term this will put a burden on all blkfronts (if 64KB pages
>> are also used by other OSes), while introducing a 64KB PV block protocol
>> will make the blkfront implementation in all OSes very similar to what
>> we have now, without replicating the splitting code amongst all the
>> possible blkfront implementations.
>>
>> Granted that some changes to blkback will be needed in order to support
>> mapping 64KB grants, but there are much fewer blkback implementations
>> out there than blkfronts.
> 
> I don't think we can rely on blkback having something in order to run
> new guests, otherwise we break compatibility: new guests won't run on
> old hypervisors.

I agree that this is far from ideal, but I don't think it's so
outrageous. For example Linux PVOPS Dom0 kernels require Xen 4.0.1 at
least in order to run, because previous versions lack the necessary
IOAPIC setup hypercall. Also, it won't prevent guests from booting, it
would just prevent them from using blkfront, but you can still get a
root filesystem using iSCSI, NFS or other means.

I'm not saying that we shouldn't take those patches, I'm just saying
that IMHO this is a workaround, and I would like to see a plan and
somebody committed to have it fixed in a proper way, by introducing a
64KB PV block protocol.

Roger.

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[PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Julien Grall <julien.grall@citrix.com> - 2015-09-07 17:40 +0200
  [PATCH v4 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux Julien Grall <julien.grall@citrix.com> - 2015-09-07 17:40 +0200
    Re: [PATCH v4 12/20] xen/balloon: Don't rely on the page granularity  is the same for Xen and Linux Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-09-07 18:50 +0200
  [PATCH v4 15/20] block/xen-blkfront: Make it running on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-09-07 17:50 +0200
  [PATCH v4 20/20] arm/xen: Add support for 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-09-07 17:50 +0200
  [PATCH v4 17/20] net/xen-netfront: Make it running on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-09-07 17:50 +0200
  [PATCH v4 10/20] xen/xenbus: Use Xen page definition Julien Grall <julien.grall@citrix.com> - 2015-09-07 17:50 +0200
  [PATCH v4 16/20] block/xen-blkback: Make it running on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-09-07 17:50 +0200
  [PATCH v4 18/20] net/xen-netback: Make it running on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-09-07 17:50 +0200
    Re: [PATCH v4 18/20] net/xen-netback: Make it running on 64KB page  granularity Wei Liu <wei.liu2@citrix.com> - 2015-09-07 19:00 +0200
      Re: [Xen-devel] [PATCH v4 18/20] net/xen-netback: Make it running on  64KB page granularity Wei Liu <wei.liu2@citrix.com> - 2015-09-08 13:10 +0200
      Re: [Xen-devel] [PATCH v4 18/20] net/xen-netback: Make it running  on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-09-08 13:10 +0200
  Re: [Xen-devel] [PATCH v4 00/20] xen/arm64: Add support for 64KB  page in Linux Julien Grall <julien.grall@citrix.com> - 2015-09-11 21:50 +0200
  Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Roger Pau Monné <roger.pau@citrix.com> - 2015-09-14 11:00 +0200
    Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Julien Grall <julien.grall@citrix.com> - 2015-09-14 12:50 +0200
      Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Roger Pau Monné <roger.pau@citrix.com> - 2015-09-14 13:10 +0200
        Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Julien Grall <julien.grall@citrix.com> - 2015-09-14 13:30 +0200
          Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Roger Pau Monné <roger.pau@citrix.com> - 2015-09-14 14:10 +0200
            Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Julien Grall <julien.grall@citrix.com> - 2015-09-14 14:50 +0200
              Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Roger Pau Monné <roger.pau@citrix.com> - 2015-09-14 16:40 +0200
                Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Julien Grall <julien.grall@citrix.com> - 2015-09-14 16:50 +0200
                Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-09-14 17:00 +0200
                Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Roger Pau Monné <roger.pau@citrix.com> - 2015-09-14 17:30 +0200
            Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Julien Grall <julien.grall@citrix.com> - 2015-09-18 16:20 +0200
        Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Arnd Bergmann <arnd@arndb.de> - 2015-09-14 13:40 +0200
          Re: [Xen-devel] [PATCH v4 00/20] xen/arm64: Add support for 64KB  page in Linux David Vrabel <david.vrabel@citrix.com> - 2015-09-15 15:20 +0200
            Re: [Xen-devel] [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux Arnd Bergmann <arnd@arndb.de> - 2015-09-15 15:30 +0200

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