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SubjectRe: [RESEND][PATCH] kvm: x86: Do not handle MMIO request in fast_page_fault
2017-09-05 18:37+0800, changbin.du@intel.com:
> From: Changbin Du <changbin.du@intel.com>
>
> If it is a MMIO request, it should be handled by slow path. This patch
> actually fixed below warning when mmu debug is enabled.
>
> WARNING: CPU: 5 PID: 2282 at arch/x86/kvm/mmu.c:226 fast_page_fault+0x41b/0x520
> CPU: 5 PID: 2282 Comm: qemu-system-x86 Not tainted 4.13.0-rc6+ #34
> task: ffff9b47f5286000 task.stack: ffffb18d03b28000
> RIP: 0010:fast_page_fault+0x41b/0x520
> Call Trace:
> tdp_page_fault+0xfb/0x290
> kvm_mmu_page_fault+0x61/0x120
> handle_ept_misconfig+0x1ba/0x1c0
> vmx_handle_exit+0xb8/0xd70
> ? kvm_arch_vcpu_ioctl_run+0x9b6/0x18e0
> kvm_arch_vcpu_ioctl_run+0xa5a/0x18e0
> ? kvm_arch_vcpu_load+0x62/0x230
> kvm_vcpu_ioctl+0x340/0x6c0
> ? kvm_vcpu_ioctl+0x340/0x6c0
> ? lock_acquire+0xf5/0x1f0
> do_vfs_ioctl+0xa2/0x670
> ? __fget+0x107/0x200
> SyS_ioctl+0x79/0x90
> entry_SYSCALL_64_fastpath+0x23/0xc2
>
> Signed-off-by: Changbin Du <changbin.du@intel.com>
> ---
> arch/x86/kvm/mmu.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 9d3f275..63c3360 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3180,6 +3180,9 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
> iterator.level < level)
> break;
>
> + if (is_mmio_spte(spte))
> + break;

Hm, handle_ept_misconfig() calls kvm_mmu_page_fault with error_code =
PFERR_RSVD_MASK. This error_code gets propagated and checked at the
beginning of page_fault_can_be_fast(), where it should break the
function execution.

How did the execution get all the way to the loop?

Thanks.

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