Re: [PATCH 2/2] KVM: Fix writeback on page boundary that propagate changes in spite of #PF

[Nadav Amit <nadav.amit@gmail.com>]

On Jan 12, 2012, at 12:27 PM, Gleb Natapov wrote:

> On Thu, Jan 12, 2012 at 12:21:00PM +0200, Avi Kivity wrote:
>> On 01/12/2012 12:12 PM, Gleb Natapov wrote:
>>> On Wed, Jan 11, 2012 at 06:53:31PM +0200, Nadav Amit wrote:
>>>> Consider the case in which an instruction emulation writeback is performed on a page boundary.
>>>> In such case, if a #PF occurs on the second page, the write to the first page already occurred and cannot be retracted.
>>>> Therefore, validation of the second page access must be performed prior to writeback.
>>>> 
>>>> Signed-off-by: Nadav Amit <nadav.amit@gmail.com>
>>>> ---
>>>> arch/x86/kvm/x86.c |   13 +++++++++++++
>>>> 1 files changed, 13 insertions(+), 0 deletions(-)
>>>> 
>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>> index 05fd3d7..7af3d67 100644
>>>> --- a/arch/x86/kvm/x86.c
>>>> +++ b/arch/x86/kvm/x86.c
>>>> @@ -3626,6 +3626,8 @@ struct read_write_emulator_ops {
>>>> 			       int bytes, void *val);
>>>> 	int (*read_write_exit_mmio)(struct kvm_vcpu *vcpu, gpa_t gpa,
>>>> 				    void *val, int bytes);
>>>> +	gpa_t (*read_write_validate)(struct kvm_vcpu *vcpu, gva_t gva,
>>>> +				     struct x86_exception *exception);
>>>> 	bool write;
>>>> };
>>>> 
>>>> @@ -3686,6 +3688,7 @@ static struct read_write_emulator_ops write_emultor = {
>>>> 	.read_write_emulate = write_emulate,
>>>> 	.read_write_mmio = write_mmio,
>>>> 	.read_write_exit_mmio = write_exit_mmio,
>>>> +	.read_write_validate = kvm_mmu_gva_to_gpa_write,
>>>> 	.write = true,
>>>> };
>>>> 
>>>> @@ -3750,6 +3753,16 @@ int emulator_read_write(struct x86_emulate_ctxt *ctxt, unsigned long addr,
>>>> 		int rc, now;
>>>> 
>>>> 		now = -addr & ~PAGE_MASK;
>>>> +
>>>> +		/* First check there is no page-fault on the next page */
>>>> +		if (ops->read_write_validate &&
>>>> +			ops->read_write_validate(vcpu, addr+now, exception) ==
>>>> +			UNMAPPED_GVA) {
>>>> +			/* #PF on the first page should be reported first */
>>>> +			ops->read_write_validate(vcpu, addr, exception);
>>>> +			return X86EMUL_PROPAGATE_FAULT;
>>>> +		}
>>>> +
>>> This undoes optimization that vcpu_mmio_gva_to_gpa() has for handling
>>> mmio. 
>> 
>> Right.  I suggest changing I/O to have two phases: first, translate the
>> virtual address into an array of two physical addresses; check
>> exceptions and report.  Then do the actual writes.
>> 
>>> Furthermore for common (non faulting) case we will check page
>>> tables twice on each write that crosses page boundary, first time here
>>> and second time in emulator_read_write_onepage().
>> 
>> Those should be very uncommon.
>> 
> Still it is better to have all the checks in one place like you suggest
> above.

I agree. I'll submit a revised version soon.
I'm just struggling with this emulation code as more stuff is broken.

Regards,
Nadav