| Message ID | 1519841208-23349-1-git-send-email-karahmed@amazon.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 28/02/2018 19:06, KarimAllah Ahmed wrote: > ... to avoid having a stale value when handling an EPT misconfig for MMIO > regions. > > MMIO regions that are not passed-through to the guest are handled through > EPT misconfigs. The first time a certain MMIO page is touched it causes an > EPT violation, then KVM marks the EPT entry to cause an EPT misconfig > instead. Any subsequent accesses to the entry will generate an EPT > misconfig. > > Things gets slightly complicated with nested guest handling for MMIO > regions that are not passed through from L0 (i.e. emulated by L0 > user-space). > > An EPT violation for one of these MMIO regions from L2, exits to L0 > hypervisor. L0 would then look at the EPT12 mapping for L1 hypervisor and > realize it is not present (or not sufficient to serve the request). Then L0 > injects an EPT violation to L1. L1 would then update its EPT mappings. The > EXIT_QUALIFICATION value for L1 would come from exit_qualification variable > in "struct vcpu". The problem is that this variable is only updated on EPT > violation and not on EPT misconfig. So if an EPT violation because of a > read happened first, then an EPT misconfig because of a write happened > afterwards. The L0 hypervisor will still contain exit_qualification value > from the previous read instead of the write and end up injecting an EPT > violation to the L1 hypervisor with an out of date EXIT_QUALIFICATION. > > The EPT violation that is injected from L0 to L1 needs to have the correct > EXIT_QUALIFICATION specially for the access bits because the individual > access bits for MMIO EPTs are updated only on actual access of this > specific type. So for the example above, the L1 hypervisor will keep > updating only the read bit in the EPT then resume the L2 guest. The L2 > guest would end up causing another exit where the L0 *again* will inject > another EPT violation to L1 hypervisor with *again* an out of date > exit_qualification which indicates a read and not a write. Then this > ping-pong just keeps happening without making any forward progress. > > The behavior of mapping MMIO regions changed in: > > commit a340b3e229b24 ("kvm: Map PFN-type memory regions as writable (if possible)") > > ... where an EPT violation for a read would also fixup the write bits to > avoid another EPT violation which by acciddent would fix the bug mentioned > above. > > This commit fixes this situation and ensures that the access bits for the > exit_qualifcation is up to date. That ensures that even L1 hypervisor > running with a KVM version before the commit mentioned above would still > work. > > ( The description above assumes EPT to be available and used by L1 > hypervisor + the L1 hypervisor is passing through the MMIO region to the L2 > guest while this MMIO region is emulated by the L0 user-space ). This looks okay. Would it be possible to add a kvm-unit-tests testcase for this? Thanks, Paolo > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Radim Krčmář <rkrcmar@redhat.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: H. Peter Anvin <hpa@zytor.com> > Cc: x86@kernel.org > Cc: kvm@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de> > --- > arch/x86/kvm/paging_tmpl.h | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h > index 5abae72..6288e9d 100644 > --- a/arch/x86/kvm/paging_tmpl.h > +++ b/arch/x86/kvm/paging_tmpl.h > @@ -452,14 +452,21 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker, > * done by is_rsvd_bits_set() above. > * > * We set up the value of exit_qualification to inject: > - * [2:0] - Derive from [2:0] of real exit_qualification at EPT violation > + * [2:0] - Derive from the access bits. The exit_qualification might be > + * out of date if it is serving an EPT misconfiguration. > * [5:3] - Calculated by the page walk of the guest EPT page tables > * [7:8] - Derived from [7:8] of real exit_qualification > * > * The other bits are set to 0. > */ > if (!(errcode & PFERR_RSVD_MASK)) { > - vcpu->arch.exit_qualification &= 0x187; > + vcpu->arch.exit_qualification &= 0x180; > + if (write_fault) > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_WRITE; > + if (user_fault) > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_READ; > + if (fetch_fault) > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_INSTR; > vcpu->arch.exit_qualification |= (pte_access & 0x7) << 3; > } > #endif >
On Fri, 2018-03-02 at 18:41 +0100, Paolo Bonzini wrote: > On 28/02/2018 19:06, KarimAllah Ahmed wrote: > > > > ... to avoid having a stale value when handling an EPT misconfig for MMIO > > regions. > > > > MMIO regions that are not passed-through to the guest are handled through > > EPT misconfigs. The first time a certain MMIO page is touched it causes an > > EPT violation, then KVM marks the EPT entry to cause an EPT misconfig > > instead. Any subsequent accesses to the entry will generate an EPT > > misconfig. > > > > Things gets slightly complicated with nested guest handling for MMIO > > regions that are not passed through from L0 (i.e. emulated by L0 > > user-space). > > > > An EPT violation for one of these MMIO regions from L2, exits to L0 > > hypervisor. L0 would then look at the EPT12 mapping for L1 hypervisor and > > realize it is not present (or not sufficient to serve the request). Then L0 > > injects an EPT violation to L1. L1 would then update its EPT mappings. The > > EXIT_QUALIFICATION value for L1 would come from exit_qualification variable > > in "struct vcpu". The problem is that this variable is only updated on EPT > > violation and not on EPT misconfig. So if an EPT violation because of a > > read happened first, then an EPT misconfig because of a write happened > > afterwards. The L0 hypervisor will still contain exit_qualification value > > from the previous read instead of the write and end up injecting an EPT > > violation to the L1 hypervisor with an out of date EXIT_QUALIFICATION. > > > > The EPT violation that is injected from L0 to L1 needs to have the correct > > EXIT_QUALIFICATION specially for the access bits because the individual > > access bits for MMIO EPTs are updated only on actual access of this > > specific type. So for the example above, the L1 hypervisor will keep > > updating only the read bit in the EPT then resume the L2 guest. The L2 > > guest would end up causing another exit where the L0 *again* will inject > > another EPT violation to L1 hypervisor with *again* an out of date > > exit_qualification which indicates a read and not a write. Then this > > ping-pong just keeps happening without making any forward progress. > > > > The behavior of mapping MMIO regions changed in: > > > > commit a340b3e229b24 ("kvm: Map PFN-type memory regions as writable (if possible)") > > > > ... where an EPT violation for a read would also fixup the write bits to > > avoid another EPT violation which by acciddent would fix the bug mentioned > > above. > > > > This commit fixes this situation and ensures that the access bits for the > > exit_qualifcation is up to date. That ensures that even L1 hypervisor > > running with a KVM version before the commit mentioned above would still > > work. > > > > ( The description above assumes EPT to be available and used by L1 > > hypervisor + the L1 hypervisor is passing through the MMIO region to the L2 > > guest while this MMIO region is emulated by the L0 user-space ). > > This looks okay. Would it be possible to add a kvm-unit-tests testcase > for this? Yup, makes sense. Just sent out a patch for kvm-unit-tests. Thanks. > > Thanks, > > Paolo > > > > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > Cc: Radim Krčmář <rkrcmar@redhat.com> > > Cc: Thomas Gleixner <tglx@linutronix.de> > > Cc: Ingo Molnar <mingo@redhat.com> > > Cc: H. Peter Anvin <hpa@zytor.com> > > Cc: x86@kernel.org > > Cc: kvm@vger.kernel.org > > Cc: linux-kernel@vger.kernel.org > > Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de> > > --- > > arch/x86/kvm/paging_tmpl.h | 11 +++++++++-- > > 1 file changed, 9 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h > > index 5abae72..6288e9d 100644 > > --- a/arch/x86/kvm/paging_tmpl.h > > +++ b/arch/x86/kvm/paging_tmpl.h > > @@ -452,14 +452,21 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker, > > * done by is_rsvd_bits_set() above. > > * > > * We set up the value of exit_qualification to inject: > > - * [2:0] - Derive from [2:0] of real exit_qualification at EPT violation > > + * [2:0] - Derive from the access bits. The exit_qualification might be > > + * out of date if it is serving an EPT misconfiguration. > > * [5:3] - Calculated by the page walk of the guest EPT page tables > > * [7:8] - Derived from [7:8] of real exit_qualification > > * > > * The other bits are set to 0. > > */ > > if (!(errcode & PFERR_RSVD_MASK)) { > > - vcpu->arch.exit_qualification &= 0x187; > > + vcpu->arch.exit_qualification &= 0x180; > > + if (write_fault) > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_WRITE; > > + if (user_fault) > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_READ; > > + if (fetch_fault) > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_INSTR; > > vcpu->arch.exit_qualification |= (pte_access & 0x7) << 3; > > } > > #endif > > > > Amazon Development Center Germany GmbH Berlin - Dresden - Aachen main office: Krausenstr. 38, 10117 Berlin Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger Ust-ID: DE289237879 Eingetragen am Amtsgericht Charlottenburg HRB 149173 B
On Sun, 2018-03-04 at 10:17 +0000, Raslan, KarimAllah wrote: > On Fri, 2018-03-02 at 18:41 +0100, Paolo Bonzini wrote: > > > > On 28/02/2018 19:06, KarimAllah Ahmed wrote: > > > > > > > > > ... to avoid having a stale value when handling an EPT misconfig for MMIO > > > regions. > > > > > > MMIO regions that are not passed-through to the guest are handled through > > > EPT misconfigs. The first time a certain MMIO page is touched it causes an > > > EPT violation, then KVM marks the EPT entry to cause an EPT misconfig > > > instead. Any subsequent accesses to the entry will generate an EPT > > > misconfig. > > > > > > Things gets slightly complicated with nested guest handling for MMIO > > > regions that are not passed through from L0 (i.e. emulated by L0 > > > user-space). > > > > > > An EPT violation for one of these MMIO regions from L2, exits to L0 > > > hypervisor. L0 would then look at the EPT12 mapping for L1 hypervisor and > > > realize it is not present (or not sufficient to serve the request). Then L0 > > > injects an EPT violation to L1. L1 would then update its EPT mappings. The > > > EXIT_QUALIFICATION value for L1 would come from exit_qualification variable > > > in "struct vcpu". The problem is that this variable is only updated on EPT > > > violation and not on EPT misconfig. So if an EPT violation because of a > > > read happened first, then an EPT misconfig because of a write happened > > > afterwards. The L0 hypervisor will still contain exit_qualification value > > > from the previous read instead of the write and end up injecting an EPT > > > violation to the L1 hypervisor with an out of date EXIT_QUALIFICATION. > > > > > > The EPT violation that is injected from L0 to L1 needs to have the correct > > > EXIT_QUALIFICATION specially for the access bits because the individual > > > access bits for MMIO EPTs are updated only on actual access of this > > > specific type. So for the example above, the L1 hypervisor will keep > > > updating only the read bit in the EPT then resume the L2 guest. The L2 > > > guest would end up causing another exit where the L0 *again* will inject > > > another EPT violation to L1 hypervisor with *again* an out of date > > > exit_qualification which indicates a read and not a write. Then this > > > ping-pong just keeps happening without making any forward progress. > > > > > > The behavior of mapping MMIO regions changed in: > > > > > > commit a340b3e229b24 ("kvm: Map PFN-type memory regions as writable (if possible)") > > > > > > ... where an EPT violation for a read would also fixup the write bits to > > > avoid another EPT violation which by acciddent would fix the bug mentioned > > > above. > > > > > > This commit fixes this situation and ensures that the access bits for the > > > exit_qualifcation is up to date. That ensures that even L1 hypervisor > > > running with a KVM version before the commit mentioned above would still > > > work. > > > > > > ( The description above assumes EPT to be available and used by L1 > > > hypervisor + the L1 hypervisor is passing through the MMIO region to the L2 > > > guest while this MMIO region is emulated by the L0 user-space ). > > > > This looks okay. Would it be possible to add a kvm-unit-tests testcase > > for this? > > Yup, makes sense. Just sent out a patch for kvm-unit-tests. Was the kvm-unit-test that I posted sufficient? > > Thanks. > > > > > > > Thanks, > > > > Paolo > > > > > > > > > > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > > Cc: Radim Krčmář <rkrcmar@redhat.com> > > > Cc: Thomas Gleixner <tglx@linutronix.de> > > > Cc: Ingo Molnar <mingo@redhat.com> > > > Cc: H. Peter Anvin <hpa@zytor.com> > > > Cc: x86@kernel.org > > > Cc: kvm@vger.kernel.org > > > Cc: linux-kernel@vger.kernel.org > > > Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de> > > > --- > > > arch/x86/kvm/paging_tmpl.h | 11 +++++++++-- > > > 1 file changed, 9 insertions(+), 2 deletions(-) > > > > > > diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h > > > index 5abae72..6288e9d 100644 > > > --- a/arch/x86/kvm/paging_tmpl.h > > > +++ b/arch/x86/kvm/paging_tmpl.h > > > @@ -452,14 +452,21 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker, > > > * done by is_rsvd_bits_set() above. > > > * > > > * We set up the value of exit_qualification to inject: > > > - * [2:0] - Derive from [2:0] of real exit_qualification at EPT violation > > > + * [2:0] - Derive from the access bits. The exit_qualification might be > > > + * out of date if it is serving an EPT misconfiguration. > > > * [5:3] - Calculated by the page walk of the guest EPT page tables > > > * [7:8] - Derived from [7:8] of real exit_qualification > > > * > > > * The other bits are set to 0. > > > */ > > > if (!(errcode & PFERR_RSVD_MASK)) { > > > - vcpu->arch.exit_qualification &= 0x187; > > > + vcpu->arch.exit_qualification &= 0x180; > > > + if (write_fault) > > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_WRITE; > > > + if (user_fault) > > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_READ; > > > + if (fetch_fault) > > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_INSTR; > > > vcpu->arch.exit_qualification |= (pte_access & 0x7) << 3; > > > } > > > #endif > > > > > > > Amazon Development Center Germany GmbH Berlin - Dresden - Aachen main office: Krausenstr. 38, 10117 Berlin Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger Ust-ID: DE289237879 Eingetragen am Amtsgericht Charlottenburg HRB 149173 B
On Mon, 2018-03-12 at 08:52 +0000, Raslan, KarimAllah wrote: > On Sun, 2018-03-04 at 10:17 +0000, Raslan, KarimAllah wrote: > > > > On Fri, 2018-03-02 at 18:41 +0100, Paolo Bonzini wrote: > > > > > > > > > On 28/02/2018 19:06, KarimAllah Ahmed wrote: > > > > > > > > > > > > > > > > ... to avoid having a stale value when handling an EPT misconfig for MMIO > > > > regions. > > > > > > > > MMIO regions that are not passed-through to the guest are handled through > > > > EPT misconfigs. The first time a certain MMIO page is touched it causes an > > > > EPT violation, then KVM marks the EPT entry to cause an EPT misconfig > > > > instead. Any subsequent accesses to the entry will generate an EPT > > > > misconfig. > > > > > > > > Things gets slightly complicated with nested guest handling for MMIO > > > > regions that are not passed through from L0 (i.e. emulated by L0 > > > > user-space). > > > > > > > > An EPT violation for one of these MMIO regions from L2, exits to L0 > > > > hypervisor. L0 would then look at the EPT12 mapping for L1 hypervisor and > > > > realize it is not present (or not sufficient to serve the request). Then L0 > > > > injects an EPT violation to L1. L1 would then update its EPT mappings. The > > > > EXIT_QUALIFICATION value for L1 would come from exit_qualification variable > > > > in "struct vcpu". The problem is that this variable is only updated on EPT > > > > violation and not on EPT misconfig. So if an EPT violation because of a > > > > read happened first, then an EPT misconfig because of a write happened > > > > afterwards. The L0 hypervisor will still contain exit_qualification value > > > > from the previous read instead of the write and end up injecting an EPT > > > > violation to the L1 hypervisor with an out of date EXIT_QUALIFICATION. > > > > > > > > The EPT violation that is injected from L0 to L1 needs to have the correct > > > > EXIT_QUALIFICATION specially for the access bits because the individual > > > > access bits for MMIO EPTs are updated only on actual access of this > > > > specific type. So for the example above, the L1 hypervisor will keep > > > > updating only the read bit in the EPT then resume the L2 guest. The L2 > > > > guest would end up causing another exit where the L0 *again* will inject > > > > another EPT violation to L1 hypervisor with *again* an out of date > > > > exit_qualification which indicates a read and not a write. Then this > > > > ping-pong just keeps happening without making any forward progress. > > > > > > > > The behavior of mapping MMIO regions changed in: > > > > > > > > commit a340b3e229b24 ("kvm: Map PFN-type memory regions as writable (if possible)") > > > > > > > > ... where an EPT violation for a read would also fixup the write bits to > > > > avoid another EPT violation which by acciddent would fix the bug mentioned > > > > above. > > > > > > > > This commit fixes this situation and ensures that the access bits for the > > > > exit_qualifcation is up to date. That ensures that even L1 hypervisor > > > > running with a KVM version before the commit mentioned above would still > > > > work. > > > > > > > > ( The description above assumes EPT to be available and used by L1 > > > > hypervisor + the L1 hypervisor is passing through the MMIO region to the L2 > > > > guest while this MMIO region is emulated by the L0 user-space ). > > > > > > This looks okay. Would it be possible to add a kvm-unit-tests testcase > > > for this? > > > > Yup, makes sense. Just sent out a patch for kvm-unit-tests. > > Was the kvm-unit-test that I posted sufficient? Never mind, I just noticed that Radim already pulled this fix to the kvm/queue. Thanks Radim :) > > > > > > > Thanks. > > > > > > > > > > > > > > Thanks, > > > > > > Paolo > > > > > > > > > > > > > > > > > > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > > > Cc: Radim Krčmář <rkrcmar@redhat.com> > > > > Cc: Thomas Gleixner <tglx@linutronix.de> > > > > Cc: Ingo Molnar <mingo@redhat.com> > > > > Cc: H. Peter Anvin <hpa@zytor.com> > > > > Cc: x86@kernel.org > > > > Cc: kvm@vger.kernel.org > > > > Cc: linux-kernel@vger.kernel.org > > > > Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de> > > > > --- > > > > arch/x86/kvm/paging_tmpl.h | 11 +++++++++-- > > > > 1 file changed, 9 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h > > > > index 5abae72..6288e9d 100644 > > > > --- a/arch/x86/kvm/paging_tmpl.h > > > > +++ b/arch/x86/kvm/paging_tmpl.h > > > > @@ -452,14 +452,21 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker, > > > > * done by is_rsvd_bits_set() above. > > > > * > > > > * We set up the value of exit_qualification to inject: > > > > - * [2:0] - Derive from [2:0] of real exit_qualification at EPT violation > > > > + * [2:0] - Derive from the access bits. The exit_qualification might be > > > > + * out of date if it is serving an EPT misconfiguration. > > > > * [5:3] - Calculated by the page walk of the guest EPT page tables > > > > * [7:8] - Derived from [7:8] of real exit_qualification > > > > * > > > > * The other bits are set to 0. > > > > */ > > > > if (!(errcode & PFERR_RSVD_MASK)) { > > > > - vcpu->arch.exit_qualification &= 0x187; > > > > + vcpu->arch.exit_qualification &= 0x180; > > > > + if (write_fault) > > > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_WRITE; > > > > + if (user_fault) > > > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_READ; > > > > + if (fetch_fault) > > > > + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_INSTR; > > > > vcpu->arch.exit_qualification |= (pte_access & 0x7) << 3; > > > > } > > > > #endif > > > > > > > > > > Amazon Development Center Germany GmbH Berlin - Dresden - Aachen main office: Krausenstr. 38, 10117 Berlin Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger Ust-ID: DE289237879 Eingetragen am Amtsgericht Charlottenburg HRB 149173 B
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h index 5abae72..6288e9d 100644 --- a/arch/x86/kvm/paging_tmpl.h +++ b/arch/x86/kvm/paging_tmpl.h @@ -452,14 +452,21 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker, * done by is_rsvd_bits_set() above. * * We set up the value of exit_qualification to inject: - * [2:0] - Derive from [2:0] of real exit_qualification at EPT violation + * [2:0] - Derive from the access bits. The exit_qualification might be + * out of date if it is serving an EPT misconfiguration. * [5:3] - Calculated by the page walk of the guest EPT page tables * [7:8] - Derived from [7:8] of real exit_qualification * * The other bits are set to 0. */ if (!(errcode & PFERR_RSVD_MASK)) { - vcpu->arch.exit_qualification &= 0x187; + vcpu->arch.exit_qualification &= 0x180; + if (write_fault) + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_WRITE; + if (user_fault) + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_READ; + if (fetch_fault) + vcpu->arch.exit_qualification |= EPT_VIOLATION_ACC_INSTR; vcpu->arch.exit_qualification |= (pte_access & 0x7) << 3; } #endif
... to avoid having a stale value when handling an EPT misconfig for MMIO regions. MMIO regions that are not passed-through to the guest are handled through EPT misconfigs. The first time a certain MMIO page is touched it causes an EPT violation, then KVM marks the EPT entry to cause an EPT misconfig instead. Any subsequent accesses to the entry will generate an EPT misconfig. Things gets slightly complicated with nested guest handling for MMIO regions that are not passed through from L0 (i.e. emulated by L0 user-space). An EPT violation for one of these MMIO regions from L2, exits to L0 hypervisor. L0 would then look at the EPT12 mapping for L1 hypervisor and realize it is not present (or not sufficient to serve the request). Then L0 injects an EPT violation to L1. L1 would then update its EPT mappings. The EXIT_QUALIFICATION value for L1 would come from exit_qualification variable in "struct vcpu". The problem is that this variable is only updated on EPT violation and not on EPT misconfig. So if an EPT violation because of a read happened first, then an EPT misconfig because of a write happened afterwards. The L0 hypervisor will still contain exit_qualification value from the previous read instead of the write and end up injecting an EPT violation to the L1 hypervisor with an out of date EXIT_QUALIFICATION. The EPT violation that is injected from L0 to L1 needs to have the correct EXIT_QUALIFICATION specially for the access bits because the individual access bits for MMIO EPTs are updated only on actual access of this specific type. So for the example above, the L1 hypervisor will keep updating only the read bit in the EPT then resume the L2 guest. The L2 guest would end up causing another exit where the L0 *again* will inject another EPT violation to L1 hypervisor with *again* an out of date exit_qualification which indicates a read and not a write. Then this ping-pong just keeps happening without making any forward progress. The behavior of mapping MMIO regions changed in: commit a340b3e229b24 ("kvm: Map PFN-type memory regions as writable (if possible)") ... where an EPT violation for a read would also fixup the write bits to avoid another EPT violation which by acciddent would fix the bug mentioned above. This commit fixes this situation and ensures that the access bits for the exit_qualifcation is up to date. That ensures that even L1 hypervisor running with a KVM version before the commit mentioned above would still work. ( The description above assumes EPT to be available and used by L1 hypervisor + the L1 hypervisor is passing through the MMIO region to the L2 guest while this MMIO region is emulated by the L0 user-space ). Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Radim Krčmář <rkrcmar@redhat.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: x86@kernel.org Cc: kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de> --- arch/x86/kvm/paging_tmpl.h | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)