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 <dgilb...@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuzn...@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 <vkuzn...@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
