Re: [PATCH v3 0/2] MTE support for KVM guest

[Andrew Jones]

On Fri, Oct 02, 2020 at 04:38:11PM +0100, Steven Price wrote:
> On 02/10/2020 15:36, Andrew Jones wrote:
> > On Fri, Sep 25, 2020 at 10:36:05AM +0100, Steven Price wrote:
> > > Version 3 of adding MTE support for KVM guests. See the previous (v2)
> > > posting for background:
> > > 
> > >   https://lore.kernel.org/r/20200904160018.29481-1-steven.price%40arm.com
> > > 
> > > These patches add support to KVM to enable MTE within a guest. They are
> > > based on Catalin's v9 MTE user-space support series[1] (currently in
> > > next).
> > > 
> > > Changes since v2:
> > > 
> > >   * MTE is no longer a VCPU feature, instead it is a VM cap.
> > > 
> > >   * Being a VM cap means easier probing (check for KVM_CAP_ARM_MTE).
> > > 
> > >   * The cap must be set before any VCPUs are created, preventing any
> > >     shenanigans where MTE is enabled for the guest after memory accesses
> > >     have been performed.
> > > 
> > > [1] https://lore.kernel.org/r/20200904103029.32083-1-catalin.marinas@xxxxxxx
> > > 
> > > Steven Price (2):
> > >    arm64: kvm: Save/restore MTE registers
> > >    arm64: kvm: Introduce MTE VCPU feature
> > > 
> > >   arch/arm64/include/asm/kvm_emulate.h       |  3 +++
> > >   arch/arm64/include/asm/kvm_host.h          |  7 +++++++
> > >   arch/arm64/include/asm/sysreg.h            |  3 ++-
> > >   arch/arm64/kvm/arm.c                       |  9 +++++++++
> > >   arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 14 ++++++++++++++
> > >   arch/arm64/kvm/mmu.c                       | 15 +++++++++++++++
> > >   arch/arm64/kvm/sys_regs.c                  | 20 +++++++++++++++-----
> > >   include/uapi/linux/kvm.h                   |  1 +
> > >   8 files changed, 66 insertions(+), 6 deletions(-)
> > > 
> > > -- 
> > > 2.20.1
> > > 
> > > 
> > 
> > Hi Steven,
> > 
> > These patches look fine to me, but I'd prefer we have a working
> > implementation in QEMU before we get too excited about the KVM
> > bits. kvmtool isn't sufficient since it doesn't support migration
> > (at least afaik). In the past we've implemented features in KVM
> > that look fine, but then issues have been discovered when trying
> > to enable them from QEMU, where we also support migration. This
> > feature looks like there's risk of issues with the userspace side.
> > Although these two patches would probably stay the same, even if
> > userspace requires more support.
> 
> I agree kvmtool isn't a great test because it doesn't support migration. The
> support in this series is just the basic support for MTE in a guest and we'd
> need to wait for the QEMU implementation before deciding whether we need any
> extra support (e.g. kernel interfaces for reading/writing tags as discussed
> before).
> 
> However, I don't think there's much danger of the support in this series
> changing - so extra support can be added when/if it's needed, but I don't
> think we need to block these series on that - QEMU can just probe for
> whatever additional support it needs before enabling MTE in a guest. I plan
> to rebase/repost after -rc1 when the user space support has been merged.
> 

Fair enough, but it feels like we'll be merging half a feature, leaving
the other half for somebody else to pick up later.

Thanks,
drew