Re: [Qemu-devel] [PATCH] kvm: Avoid COW if KVM MMU is asynchronous

[Avi Kivity]

Jan Kiszka wrote:
> If the KVM MMU is asynchronous (kernel does not support MMU_NOTIFIER),
> we have to avoid COW for the guest memory. Otherwise we risk serious
> breakage when guest pages change there physical locations due to COW
> after fork. Seen when forking smbd during runtime via -smb.
>
> Signed-off-by: Jan Kiszka <address@hidden>
> ---
>
>  exec.c |   14 ++++++++++++++
>  1 files changed, 14 insertions(+), 0 deletions(-)
>
> diff --git a/exec.c b/exec.c
> index 23013fb..ce0603e 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2477,6 +2477,20 @@ ram_addr_t qemu_ram_alloc(ram_addr_t size)
>      new_block->next = ram_blocks;
>      ram_blocks = new_block;
>  
> +    if (kvm_enabled() && !kvm_has_sync_mmu()) {
> +#ifdef MADV_DONTFORK
> +        int ret = madvise(new_block->host, size, MADV_DONTFORK);
> +        if (ret) {
> +            perror("madvice");
> +            exit(1);
> +        }
> +#else
> +        fprintf(stderr,
> +                "Need MADV_DONTFORK in absence of synchronous KVM MMU\n");
> +        exit(1);
> +#endif
> +    }
> +

Suggest wrapping in a function and hiding it deep inside kvm-all.c.

--
error compiling committee.c: too many arguments to function