linux-kernel - Re: [RESEND][PATCH] kvm: x86: Do not handle MMIO request in fast_page

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Message-ID: <20170914035543.GA32034@intel.com>
Date:   Thu, 14 Sep 2017 11:55:43 +0800
From:   "Du, Changbin" <changbin.du@...el.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     changbin.du@...el.com, pbonzini@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND][PATCH] kvm: x86: Do not handle MMIO request in
 fast_page_fault

On Wed, Sep 13, 2017 at 04:39:56PM +0200, Radim Krčmář wrote:
> 2017-09-05 18:37+0800, changbin.du@...el.com:
> > From: Changbin Du <changbin.du@...el.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@...el.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?
> 
hmm, a recent Paolo's cleanup patch already fixed it.
e08d26f ("KVM: x86: simplify ept_misconfig")

In the past, PFERR_RSVD_MASK is not set.

So this patch doesnt need any more. thanks.

> Thanks.

-- 
Thanks,
Changbin Du

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