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Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory trans
From: |
David Hildenbrand |
Subject: |
Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions |
Date: |
Mon, 26 Oct 2020 11:43:22 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 |
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.
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
--
Thanks,
David / dhildenb
- [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, Vitaly Kuznetsov, 2020/10/26
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions,
David Hildenbrand <=
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, David Hildenbrand, 2020/10/26
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, Vitaly Kuznetsov, 2020/10/27
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, David Hildenbrand, 2020/10/27
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, Vitaly Kuznetsov, 2020/10/27
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, David Hildenbrand, 2020/10/27
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, Vitaly Kuznetsov, 2020/10/27
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, David Hildenbrand, 2020/10/27
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, Vitaly Kuznetsov, 2020/10/27
- Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions, Igor Mammedov, 2020/10/27