linux-kernel - Re: [PATCH v5 02/12] Add PV MSR to enable asynchronous page faults delivery.

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Date:	Mon, 23 Aug 2010 18:29:17 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, hpa@...or.com,
	riel@...hat.com, cl@...ux-foundation.org, mtosatti@...hat.com
Subject: Re: [PATCH v5 02/12] Add PV MSR to enable asynchronous page faults
 delivery.

On Mon, Aug 23, 2010 at 06:22:02PM +0300, Avi Kivity wrote:
>  On 07/19/2010 06:30 PM, Gleb Natapov wrote:
> >Guess enables async PF vcpu functionality using this MSR.
> >
> >
> >
> >+static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
> >+{
> >+	u64 gpa = data&  ~0x3f;
> >+	int offset = offset_in_page(gpa);
> >+	unsigned long addr;
> >+
> >+	/* Bits 1:5 are resrved, Should be zero */
> >+	if (data&  0x3e)
> >+		return 1;
> >+
> >+	vcpu->arch.apf_msr_val = data;
> >+
> >+	if (!(data&  KVM_ASYNC_PF_ENABLED)) {
> >+		vcpu->arch.apf_data = NULL;
> >+		return 0;
> >+	}
> >+
> >+	addr = gfn_to_hva(vcpu->kvm, gpa>>  PAGE_SHIFT);
> >+	if (kvm_is_error_hva(addr))
> >+		return 1;
> >+
> >+	vcpu->arch.apf_data = (u32 __user*)(addr + offset);
> 
> This can be invalidated by host userspace playing with memory
> regions.  It needs to be recalculated on memory map changes, and it
> may disappear from under the guest's feet (in which case we're
> allowed to KVM_REQ_TRIPLE_FAULT it).
> 
> (note: this is a much better approach than kvmclock's and vapic's,
> we should copy it there)
> 
apf_put_user() tracks memory slot changes and revalidate the address if
needed.

> >+
> >+	/* check if address is mapped */
> >+	if (get_user(offset, vcpu->arch.apf_data)) {
> >+		vcpu->arch.apf_data = NULL;
> >+		return 1;
> >+	}
> 
> So, this check can succeed today but fail tomorrow.
> 

> >+	return 0;
> >+}
> >+
> 
> -- 
> error compiling committee.c: too many arguments to function

--
			Gleb.
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