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Re: [Qemu-devel] [PATCH v0 0/7] Background snapshots
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v0 0/7] Background snapshots |
Date: |
Wed, 25 Jul 2018 20:17:37 +0100 |
User-agent: |
Mutt/1.10.0 (2018-05-17) |
* Peter Xu (address@hidden) wrote:
> On Fri, Jun 29, 2018 at 12:53:59PM +0100, Dr. David Alan Gilbert wrote:
> > * Denis Plotnikov (address@hidden) wrote:
> > > The patch set adds the ability to make external snapshots while VM is
> > > running.
> >
> > cc'ing in Andrea since this uses sigsegv's to avoid userfault-wp that
> > isn't there yet.
> >
> > Hi Denis,
> > How robust are you finding this SEGV based trick; for example what
> > about things like the kernel walking vhost queues or similar kernel
> > nasties?
>
> (I'm commenting on this old series to keep the discussion together)
>
> If we want to make this series really work for people, we should
> possibly need to know whether it could work with vhost (otherwise we
> might need to go back to userfaultfd write-protection).
>
> I digged a bit on the vhost-net IO, it should be using two ways to
> write to guest memory:
>
> - copy_to_user(): this should possibly still be able to be captured by
> mprotect() (after some confirmation from Paolo, but still we'd
> better try it out)
What confuses me here is who is going to get the signal from this and
how we recover from the signal - or does it come back as an error
on the vhost fd somehow?
Dave
> - kmap_atomic(): this is used to log dirty pages, this seems to be
> incompatible with mprotect() but I think we can just make sure dirty
> page tracking is disabled when live snapshot is enabled (please
> refer to the code clip below as a referece)
>
> So IMHO this series should work with vhost if with logging off, but we
> need to confirm... Denis, do you want to do some more test with vhost
> when you do your next post (possibly with changes like below)?
>
> =====================
>
> diff --git a/migration/ram.c b/migration/ram.c
> index 24dea2730c..a3fa256143 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1605,6 +1605,10 @@ static void migration_bitmap_sync(RAMState *rs)
> int64_t end_time;
> uint64_t bytes_xfer_now;
>
> + if (live_snapshot) {
> + return;
> + }
> +
> ram_counters.dirty_sync_count++;
>
> if (!rs->time_last_bitmap_sync) {
> @@ -2952,7 +2956,10 @@ static void ram_init_bitmaps(RAMState *rs)
> rcu_read_lock();
>
> ram_list_init_bitmaps();
> - memory_global_dirty_log_start();
> +
> + if (!live_snapshot) {
> + memory_global_dirty_log_start();
> + }
> migration_bitmap_sync(rs);
>
> rcu_read_unlock();
>
>
> >
> > Dave
> >
> > > The workflow to make a snapshot is the following:
> > > 1. Pause the vm
> > > 2. Make a snapshot of block devices using the scheme of your choice
> > > 3. Turn on background-snapshot migration capability
> > > 4. Start the migration using the destination (migration stream) of your
> > > choice.
> > > The migration will resume the vm execution by itself
> > > when it has the devices' states saved and is ready to start ram
> > > writing
> > > to the migration stream.
> > > 5. Listen to the migration finish event
> > >
> > > The feature relies on KVM unapplied ability to report the faulting
> > > address.
> > > Please find the KVM patch snippet to make the patchset work below:
> > >
> > > +++ b/arch/x86/kvm/vmx.c
> > > @@ -XXXX,X +XXXX,XX @@ static int handle_ept_violation(struct kvm_vcpu
> > > *vcpu)
> > >
> > > vcpu->arch.exit_qualification = exit_qualification;
> > >
> > > - return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
> > > + r = kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
> > > + if (r == -EFAULT) {
> > > + unsigned long hva = kvm_vcpu_gfn_to_hva(vcpu, gpa >>
> > > PAGE_SHIFT);
> > > +
> > > + vcpu->run->exit_reason = KVM_EXIT_FAIL_MEM_ACCESS;
> > > + vcpu->run->hw.hardware_exit_reason =
> > > EXIT_REASON_EPT_VIOLATION;
> > > + vcpu->run->fail_mem_access.hva = hva | (gpa &
> > > (PAGE_SIZE-1));
> > > + r = 0;
> > > +
> > > + }
> > > + return r;
> > >
> > > The patch to KVM can be sent if the patch set approved
> > >
> > > Denis Plotnikov (7):
> > > migration: add background snapshot capability
> > > bitops: add some atomic versions of bitmap operations
> > > threads: add infrastructure to process sigsegv
> > > migration: add background snapshot infrastructure
> > > kvm: add failed memeory access exit reason
> > > kvm: add vCPU failed memeory access processing
> > > migration: add background snapshotting
> > >
> > > include/exec/ram_addr.h | 7 +
> > > include/exec/ramlist.h | 4 +-
> > > include/qemu/bitops.h | 24 +++
> > > include/qemu/thread.h | 5 +
> > > linux-headers/linux/kvm.h | 5 +
> > > migration/migration.c | 141 +++++++++++++++-
> > > migration/migration.h | 1 +
> > > migration/ram.c | 333 ++++++++++++++++++++++++++++++++++++--
> > > migration/ram.h | 11 +-
> > > migration/savevm.c | 91 ++++++-----
> > > migration/savevm.h | 2 +
> > > qapi/migration.json | 6 +-
> > > target/i386/kvm.c | 18 +++
> > > util/qemu-thread-posix.c | 50 ++++++
> > > 14 files changed, 635 insertions(+), 63 deletions(-)
> > >
> > > --
> > > 2.17.0
> > >
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> >
>
> Regards,
>
> --
> Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK