What's the difference between EPT_MISCONFIG and EPT_VIOLATION?

What's the difference between EPT_MISCONFIG and EPT_VIOLATION?

[Zhang Haoyu]

Hi,

EXIT_REASON_EPT_VIOLATION's corresponding handle is handle_ept_violation(),
and EXIT_REASON_EPT_MISCONFIG's corresponding handle is handle_ept_misconfig(),
what's the difference between them?

I read the SDM-3C 28.2.3 EPT-Induced VM Exits, and found below description,
"An EPT misconfiguration occurs when, in the course of translating 
a guest-physical address, the logical processor encounters an EPT 
paging-structure entry that contains an unsupported value. An EPT 
violation occurs when there is no EPT misconfiguration but the EPT 
paging-structure entries disallow an access using the guest physical
address."

According to above description, EPT-MISCONFIG is from error settings ,
but from the its exit-handle handle_ept_misconfig(),
it seems that handle_ept_misconfig() handles mmio pagefault,
I'm really confused, I think I'm missing something,
any advices?

Thanks,
Zhang Haoyu

Re: What's the difference between EPT_MISCONFIG and EPT_VIOLATION?

[Gleb Natapov]

On Wed, Dec 03, 2014 at 05:50:33PM +0800, Zhang Haoyu wrote:
> Hi,
> 
> EXIT_REASON_EPT_VIOLATION's corresponding handle is handle_ept_violation(),
> and EXIT_REASON_EPT_MISCONFIG's corresponding handle is handle_ept_misconfig(),
> what's the difference between them?
> 
> I read the SDM-3C 28.2.3 EPT-Induced VM Exits, and found below description,
> "An EPT misconfiguration occurs when, in the course of translating 
> a guest-physical address, the logical processor encounters an EPT 
> paging-structure entry that contains an unsupported value. An EPT 
> violation occurs when there is no EPT misconfiguration but the EPT 
> paging-structure entries disallow an access using the guest physical
> address."
> 
> According to above description, EPT-MISCONFIG is from error settings ,
> but from the its exit-handle handle_ept_misconfig(),
> it seems that handle_ept_misconfig() handles mmio pagefault,
> I'm really confused, I think I'm missing something,
> any advices?
> 
EXIT_REASON_EPT_VIOLATION is similar to a "page not present" pagefault
EXIT_REASON_EPT_MISCONFIG is similar to a "reserved bit set" pagefault.
handle_ept_misconfig() handles mmio pagefault because KVM has an
optimization that uses reserved bits to mark mmio regions.

--
			Gleb.

Re: What's the difference between EPT_MISCONFIG and EPT_VIOLATION?

[Zhang Haoyu]

> > Hi,
> > 
> > EXIT_REASON_EPT_VIOLATION's corresponding handle is handle_ept_violation(),
> > and EXIT_REASON_EPT_MISCONFIG's corresponding handle is handle_ept_misconfig(),
> > what's the difference between them?
> > 
> > I read the SDM-3C 28.2.3 EPT-Induced VM Exits, and found below description,
> > "An EPT misconfiguration occurs when, in the course of translating 
> > a guest-physical address, the logical processor encounters an EPT 
> > paging-structure entry that contains an unsupported value. An EPT 
> > violation occurs when there is no EPT misconfiguration but the EPT 
> > paging-structure entries disallow an access using the guest physical
> > address."
> > 
> > According to above description, EPT-MISCONFIG is from error settings ,
> > but from the its exit-handle handle_ept_misconfig(),
> > it seems that handle_ept_misconfig() handles mmio pagefault,
> > I'm really confused, I think I'm missing something,
> > any advices?
> > 
> EXIT_REASON_EPT_VIOLATION is similar to a "page not present" pagefault
> EXIT_REASON_EPT_MISCONFIG is similar to a "reserved bit set" pagefault.
> handle_ept_misconfig() handles mmio pagefault because KVM has an
> optimization that uses reserved bits to mark mmio regions.
>
Thanks, Gleb, 
where does kvm use the reserved bits to mark mmio regions?

> --
> 			Gleb.

Re: What's the difference between EPT_MISCONFIG and EPT_VIOLATION?

[Gleb Natapov]

On Wed, Dec 03, 2014 at 06:12:10PM +0800, Zhang Haoyu wrote:
> > > Hi,
> > > 
> > > EXIT_REASON_EPT_VIOLATION's corresponding handle is handle_ept_violation(),
> > > and EXIT_REASON_EPT_MISCONFIG's corresponding handle is handle_ept_misconfig(),
> > > what's the difference between them?
> > > 
> > > I read the SDM-3C 28.2.3 EPT-Induced VM Exits, and found below description,
> > > "An EPT misconfiguration occurs when, in the course of translating 
> > > a guest-physical address, the logical processor encounters an EPT 
> > > paging-structure entry that contains an unsupported value. An EPT 
> > > violation occurs when there is no EPT misconfiguration but the EPT 
> > > paging-structure entries disallow an access using the guest physical
> > > address."
> > > 
> > > According to above description, EPT-MISCONFIG is from error settings ,
> > > but from the its exit-handle handle_ept_misconfig(),
> > > it seems that handle_ept_misconfig() handles mmio pagefault,
> > > I'm really confused, I think I'm missing something,
> > > any advices?
> > > 
> > EXIT_REASON_EPT_VIOLATION is similar to a "page not present" pagefault
> > EXIT_REASON_EPT_MISCONFIG is similar to a "reserved bit set" pagefault.
> > handle_ept_misconfig() handles mmio pagefault because KVM has an
> > optimization that uses reserved bits to mark mmio regions.
> >
> Thanks, Gleb, 
> where does kvm use the reserved bits to mark mmio regions?
> 
arch/x86/kvm/mmu.c:mark_mmio_spte

--
			Gleb.

Re: What's the difference between EPT_MISCONFIG and EPT_VIOLATION?

[Paolo Bonzini]

On 03/12/2014 11:12, Zhang Haoyu wrote:
>> > EXIT_REASON_EPT_VIOLATION is similar to a "page not present" pagefault
>> > EXIT_REASON_EPT_MISCONFIG is similar to a "reserved bit set" pagefault.
>> > handle_ept_misconfig() handles mmio pagefault because KVM has an
>> > optimization that uses reserved bits to mark mmio regions.
>> >
> Thanks, Gleb, 
> where does kvm use the reserved bits to mark mmio regions?

ept_set_mmio_spte_mask is where KVM tells mmu.c how to mark MMIO
regions.  You can search mmu.c for shadow_mmio_mask and is_mmio_spte in
order to find out more about this optimization, you'll also get to the
mark_mmio_spte function that Gleb mentioned.

Paolo