Re: [PATCH 2/5] KVM: Make locked operations truly atomic

[Jan Kiszka <jan.kiszka@web.de>]

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Avi Kivity wrote:
> Once upon a time, locked operations were emulated while holding the mmu mutex.
> Since mmu pages were write protected, it was safe to emulate the writes in
> a non-atomic manner, since there could be no other writer, either in the
> guest or in the kernel.
> 
> These days emulation takes place without holding the mmu spinlock, so the
> write could be preempted by an unshadowing event, which exposes the page
> to writes by the guest.  This may cause corruption of guest page tables.
> 
> Fix by using an atomic cmpxchg for these operations.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  arch/x86/kvm/x86.c |   69 ++++++++++++++++++++++++++++++++++++----------------
>  1 files changed, 48 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9d02cc7..d724a52 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3299,41 +3299,68 @@ int emulator_write_emulated(unsigned long addr,
>  }
>  EXPORT_SYMBOL_GPL(emulator_write_emulated);
>  
> +#define CMPXCHG_TYPE(t, ptr, old, new) \
> +	(cmpxchg((t *)(ptr), *(t *)(old), *(t *)(new)) == *(t *)(old))
> +
> +#ifdef CONFIG_X86_64
> +#  define CMPXCHG64(ptr, old, new) CMPXCHG_TYPE(u64, ptr, old, new)
> +#else
> +#  define CMPXCHG64(ptr, old, new) \
> +	(cmpxchg64((u64 *)(ptr), *(u64 *)(old), *(u *)(new)) == *(u64 *)(old))
                                                ^^^^^^
This should cause the 32-bit build breakage I see with the current next
branch.

Jan


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