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@xxxxxxxxx> >>>> --- >>>> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html