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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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)
-- 
2.25.4