Re: [PATCH 1/5] KVM: x86/mmu: Don't attempt to map leaf if target TDP MMU SPTE is frozen

[Robert Hoo]

On Tue, 2022-12-13 at 03:30 +0000, Sean Christopherson wrote:
> Hoist the is_removed_spte() check above the "level == goal_level"
> check
> when walking SPTEs during a TDP MMU page fault to avoid attempting to
> map
> a leaf entry if said entry is frozen by a different task/vCPU.
> 
>   ------------[ cut here ]------------
>   WARNING: CPU: 3 PID: 939 at arch/x86/kvm/mmu/tdp_mmu.c:653
> kvm_tdp_mmu_map+0x269/0x4b0
>   Modules linked in: kvm_intel
>   CPU: 3 PID: 939 Comm: nx_huge_pages_t Not tainted 6.1.0-rc4+ #67
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0
> 02/06/2015
>   RIP: 0010:kvm_tdp_mmu_map+0x269/0x4b0
>   RSP: 0018:ffffc9000068fba8 EFLAGS: 00010246
>   RAX: 00000000000005a0 RBX: ffffc9000068fcc0 RCX: 0000000000000005
>   RDX: ffff88810741f000 RSI: ffff888107f04600 RDI: ffffc900006a3000
>   RBP: 060000010b000bf3 R08: 0000000000000000 R09: 0000000000000000
>   R10: 0000000000000000 R11: 000ffffffffff000 R12: 0000000000000005
>   R13: ffff888113670000 R14: ffff888107464958 R15: 0000000000000000
>   FS:  00007f01c942c740(0000) GS:ffff888277cc0000(0000)
> knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000000000000 CR3: 0000000117013006 CR4: 0000000000172ea0
>   Call Trace:
>    <TASK>
>    kvm_tdp_page_fault+0x10c/0x130
>    kvm_mmu_page_fault+0x103/0x680
>    vmx_handle_exit+0x132/0x5a0 [kvm_intel]
>    vcpu_enter_guest+0x60c/0x16f0
>    kvm_arch_vcpu_ioctl_run+0x1e2/0x9d0
>    kvm_vcpu_ioctl+0x271/0x660
>    __x64_sys_ioctl+0x80/0xb0
>    do_syscall_64+0x2b/0x50
>    entry_SYSCALL_64_after_hwframe+0x46/0xb0
>    </TASK>
>   ---[ end trace 0000000000000000 ]---
> 
> Fixes: 63d28a25e04c ("KVM: x86/mmu: simplify kvm_tdp_mmu_map flow
> when guest has to retry")
> Cc: Robert Hoo <robert.hu@xxxxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 764f7c87286f..b740f38fedcc 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1162,9 +1162,6 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu,
> struct kvm_page_fault *fault)
>  		if (fault->nx_huge_page_workaround_enabled)
>  			disallowed_hugepage_adjust(fault,
> iter.old_spte, iter.level);
>  
> -		if (iter.level == fault->goal_level)
> -			break;
> -
>  		/*
>  		 * If SPTE has been frozen by another thread, just give
> up and
>  		 * retry, avoiding unnecessary page table allocation
> and free.
> @@ -1172,6 +1169,9 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu,
> struct kvm_page_fault *fault)
>  		if (is_removed_spte(iter.old_spte))
>  			goto retry;
>  
> +		if (iter.level == fault->goal_level)
> +			break;
> +
>  		/* Step down into the lower level page table if it
> exists. */
>  		if (is_shadow_present_pte(iter.old_spte) &&
>  		    !is_large_pte(iter.old_spte))

Reviewed-by: Robert Hoo <robert.hu@xxxxxxxxxxxxxxx>