Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions

[David Hildenbrand]

On 27.10.20 13:36, Vitaly Kuznetsov wrote:
> David Hildenbrand <david@redhat.com> writes:
>
> > On 26.10.20 11:43, David Hildenbrand wrote:
> > > On 26.10.20 09:49, Vitaly Kuznetsov wrote:
> > > > Currently, KVM doesn't provide an API to make atomic updates to memmap when
> > > > the change touches more than one memory slot, e.g. in case we'd like to
> > > > punch a hole in an existing slot.
> > > >
> > > > Reports are that multi-CPU Q35 VMs booted with OVMF sometimes print something
> > > > like
> > > >
> > > > !!!! X64 Exception Type - 0E(#PF - Page-Fault)  CPU Apic ID - 00000003 !!!!
> > > > ExceptionData - 0000000000000010  I:1 R:0 U:0 W:0 P:0 PK:0 SS:0 SGX:0
> > > > RIP  - 000000007E35FAB6, CS  - 0000000000000038, RFLAGS - 0000000000010006
> > > > RAX  - 0000000000000000, RCX - 000000007E3598F2, RDX - 00000000078BFBFF
> > > > ...
> > > >
> > > > The problem seems to be that TSEG manipulations on one vCPU are not atomic
> > > > from other vCPUs views. In particular, here's the strace:
> > > >
> > > > Initial creation of the 'problematic' slot:
> > > >
> > > > 10085 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, 
> > > > guest_phys_addr=0x100000,
> > > >     memory_size=2146435072, userspace_addr=0x7fb89bf00000}) = 0
> > > >
> > > > ... and then the update (caused by e.g. mch_update_smram()) later:
> > > >
> > > > 10090 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, 
> > > > guest_phys_addr=0x100000,
> > > >     memory_size=0, userspace_addr=0x7fb89bf00000}) = 0
> > > > 10090 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, 
> > > > guest_phys_addr=0x100000,
> > > >     memory_size=2129657856, userspace_addr=0x7fb89bf00000}) = 0
> > > >
> > > > In case KVM has to handle any event on a different vCPU in between these
> > > > two calls the #PF will get triggered.
> > > >
> > > > An ideal solution to the problem would probably require KVM to provide a
> > > > new API to do the whole transaction in one shot but as a band-aid we can
> > > > just pause all vCPUs to make memory transations atomic.
> > > >
> > > > Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > > Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> > > > ---
> > > > RFC: Generally, memap updates happen only a few times during guest boot but
> > > > I'm not sure there are no scenarios when pausing all vCPUs is undesireable
> > > > from performance point of view. Also, I'm not sure if kvm_enabled() check
> > > > is needed.
> > > > Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> > > > ---
> > > >   softmmu/memory.c | 11 +++++++++--
> > > >   1 file changed, 9 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/softmmu/memory.c b/softmmu/memory.c
> > > > index fa280a19f7f7..0bf6f3f6d5dc 100644
> > > > --- a/softmmu/memory.c
> > > > +++ b/softmmu/memory.c
> > > > @@ -28,6 +28,7 @@
> > > >   
> > > >   #include "exec/memory-internal.h"
> > > >   #include "exec/ram_addr.h"
> > > > +#include "sysemu/cpus.h"
> > > >   #include "sysemu/kvm.h"
> > > >   #include "sysemu/runstate.h"
> > > >   #include "sysemu/tcg.h"
> > > > @@ -1057,7 +1058,9 @@ static void address_space_update_topology(AddressSpace 
> > > > *as)
> > > >   void memory_region_transaction_begin(void)
> > > >   {
> > > >       qemu_flush_coalesced_mmio_buffer();
> > > > -    ++memory_region_transaction_depth;
> > > > +    if ((++memory_region_transaction_depth == 1) && kvm_enabled()) {
> > > > +        pause_all_vcpus();
> > > > +    }
> > > >   }
> > > >   
> > > >   void memory_region_transaction_commit(void)
> > > > @@ -1087,7 +1090,11 @@ void memory_region_transaction_commit(void)
> > > >               }
> > > >               ioeventfd_update_pending = false;
> > > >           }
> > > > -   }
> > > > +
> > > > +        if (kvm_enabled()) {
> > > > +            resume_all_vcpus();
> > > > +        }
> > > > +    }
> > > >   }
> > > >   
> > > >   static void memory_region_destructor_none(MemoryRegion *mr)
> > >
> > > This is in general unsafe. pause_all_vcpus() will temporarily drop the
> > > BQL, resulting in bad things happening to caller sites.
>
> Oh, I see, thanks! I was expecting there's a reason we don't have this
> simple fix in already :-)
>
> > > I studies the involved issues quite intensively when wanting to resize
> > > memory regions from virtio-mem code. It's not that easy.
> > >
> > > Have a look at my RFC for resizing. You can apply something similar to
> > > other operations.
> > >
> > > https://www.mail-archive.com/qemu-devel@nongnu.org/msg684979.html
> >
> > Oh, and I even mentioned the case you try to fix here back then
> >
> > "
> > Instead of inhibiting during the region_resize(), we could inhibit for the
> > hole memory transaction (from begin() to commit()). This could be nice,
> > because also splitting of memory regions would be atomic (I remember there
> > was a BUG report regarding that), however, I am not sure if that might
> > impact any RT users.
> > "
> >
> > The current patches live in
> > https://github.com/davidhildenbrand/qemu/commits/virtio-mem-next
> >
> > Especially
> >
> > https://github.com/davidhildenbrand/qemu/commit/433fbb3abed20f15030e42f2b2bea7e6b9a15180
>
> I'm not sure why we're focusing on ioctls here. I was debugging my case
> quite some time ago but from what I remember it had nothing to do with
> ioctls from QEMU. When we are removing a memslot any exit to KVM may
> trigger an error condition as we'll see that vCPU or some of our
> internal structures (e.g. VMCS for a nested guest) references
> non-existent memory. I don't see a good solution other than making the
> update fully atomic from *all* vCPUs point of view and this requires
> stopping all CPUs -- either from QEMU or from KVM.

I cannot follow. My patch waits until *any* KVM ioctls are out of the 
kernel. That includes VCPUs, but also other ioctls (because there are 
some that require a consistent memory block state).

So from a KVM point of view, the CPUs are stopped.

--
Thanks,

David / dhildenb