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SubjectRe: [PATCH v5 1/9] Documentation: Introduce EPT based Subpage Protection
On Fri, Oct 11, 2019 at 01:31:08PM -0700, Jim Mattson wrote:
> On Tue, Sep 17, 2019 at 1:52 AM Yang Weijiang <weijiang.yang@intel.com> wrote:
> >
> > Co-developed-by: yi.z.zhang@linux.intel.com
> > Signed-off-by: yi.z.zhang@linux.intel.com
> > Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> > ---
> > Documentation/virtual/kvm/spp_kvm.txt | 178 ++++++++++++++++++++++++++
> > 1 file changed, 178 insertions(+)
> > create mode 100644 Documentation/virtual/kvm/spp_kvm.txt
> >
> > diff --git a/Documentation/virtual/kvm/spp_kvm.txt b/Documentation/virtual/kvm/spp_kvm.txt
> > new file mode 100644
> > index 000000000000..1bd1c11d0a99
> > --- /dev/null
> > +++ b/Documentation/virtual/kvm/spp_kvm.txt
> > @@ -0,0 +1,178 @@
> > +EPT-Based Sub-Page Protection (SPP) for KVM
> > +====================================================
> > +
> > +1.Overview
> > + EPT-based Sub-Page Protection(SPP) allows VMM to specify
> > + fine-grained(128byte per sub-page) write-protection for guest physical
> > + memory. When it's enabled, the CPU enforces write-access permission
> > + for the sub-pages within a 4KB page, if corresponding bit is set in
> > + permission vector, write to sub-page region is allowed, otherwise,
> > + it's prevented with a EPT violation.
> > +
> > + *Note*: In current implementation, SPP is exclusive with nested flag,
> > + if it's on, SPP feature won't work.
> > +
> > +2.SPP Operation
> > + Sub-Page Protection Table (SPPT) is introduced to manage sub-page
> > + write-access permission.
> > +
> > + It is active when:
> > + a) nested flag is turned off.
> > + b) "sub-page write protection" VM-execution control is 1.
> > + c) SPP is initialized with KVM_INIT_SPP ioctl.
> > + d) Sub-page permissions are set with KVM_SUBPAGES_SET_ACCESS ioctl.
> > + see below sections for details.
> > +
> > + __________________________________________________________________________
> > +
> > + How SPP hardware works:
> > + __________________________________________________________________________
> > +
> > + Guest write access --> GPA --> Walk EPT --> EPT leaf entry -----|
> > + |---------------------------------------------------------------|
> > + |-> if VMexec_control.spp && ept_leaf_entry.spp_bit (bit 61)
> > + |
> > + |-> <false> --> EPT legacy behavior
> > + |
> > + |
> > + |-> <true> --> if ept_leaf_entry.writable
> > + |
> > + |-> <true> --> Ignore SPP
> > + |
> > + |-> <false> --> GPA --> Walk SPP 4-level table--|
> > + |
> > + |------------<----------get-the-SPPT-point-from-VMCS-filed-----<------|
> /filed/field/
> > + |
> > + Walk SPP L4E table
> > + |
> > + |---> if-entry-misconfiguration ------------>-------|-------<---------|
> > + | | |
> > + else | |
> > + | | |
> > + | |------------------SPP VMexit<-----------------| |
> > + | | |
> > + | |-> exit_qualification & sppt_misconfig --> sppt misconfig |
> > + | | |
> > + | |-> exit_qualification & sppt_miss --> sppt miss |
> > + |---| |
> > + | |
> > + walk SPPT L3E--|--> if-entry-misconfiguration------------>------------|
> > + | |
> > + else |
> > + | |
> > + | |
> > + walk SPPT L2E --|--> if-entry-misconfiguration-------->-------|
> > + | |
> > + else |
> > + | |
> > + | |
> > + walk SPPT L1E --|-> if-entry-misconfiguration--->----|
> > + |
> > + else
> > + |
> > + |-> if sub-page writable
> > + |-> <true> allow, write access
> > + |-> <false> disallow, EPT violation
> > + ______________________________________________________________________________
> > +
> > +3.IOCTL Interfaces
> > +
> > + KVM_INIT_SPP:
> > + Allocate storage for sub-page permission vectors and SPPT root page.
> > +
> > + KVM_SUBPAGES_GET_ACCESS:
> > + Get sub-page write permission vectors for given continuous guest pages.
> /continuous/contiguous/
Thanks for all the corrections.

> > +
> > + KVM_SUBPAGES_SET_ACCESS
> > + Set SPP bit in EPT leaf entries for given continuous guest pages. The
> /continuous/contiguous/
> > + actual SPPT setup is triggered when SPP miss vm-exit is handled.
> > +
> > + /* for KVM_SUBPAGES_GET_ACCESS and KVM_SUBPAGES_SET_ACCESS */
> > + struct kvm_subpage_info {
> > + __u64 gfn; /* the first page gfn of the continuous pages */
> /continuous/contiguous/
> > + __u64 npages; /* number of 4K pages */
> > + __u64 *access_map; /* sub-page write-access bitmap array */
> > + };
> > +
> > + #define KVM_SUBPAGES_GET_ACCESS _IOR(KVMIO, 0x49, __u64)
> > + #define KVM_SUBPAGES_SET_ACCESS _IOW(KVMIO, 0x4a, __u64)
> > + #define KVM_INIT_SPP _IOW(KVMIO, 0x4b, __u64)
>
> The ioctls should be documented in api.txt.
>
Sure, will do it.

> > +4.Set Sub-Page Permission
> > +
> > + * To enable SPP protection, system admin sets sub-page permission via
> Why system admin? Can't any kvm user do this?
Oops, will change it.

> > + KVM_SUBPAGES_SET_ACCESS ioctl:
> > + (1) It first stores the access permissions in bitmap array.
> > +
> > + (2) Then, if the target 4KB page is mapped as PT_PAGE_TABLE_LEVEL entry in EPT,
> /page is/pages are/
> > + it sets SPP bit of the corresponding entry to mark sub-page protection.
> > + If the 4KB page is mapped as PT_DIRECTORY_LEVEL or PT_PDPE_LEVEL, it
> /page is/pages are/
> > + zapps the hugepage entry and let following memroy access to trigger EPT
> /zapps/zaps/, /entry/enttries/, /memroy/memory/
> > + page fault, there the gfn is check against SPP permission bitmap and
> /page fault/violation/
> > + proper level is selected to set up EPT entry.
> > +
> > +
> > + The SPPT paging structure format is as below:
> > +
> > + Format of the SPPT L4E, L3E, L2E:
> > + | Bit | Contents |
> > + | :----- | :------------------------------------------------------------------------|
> > + | 0 | Valid entry when set; indicates whether the entry is present |
> > + | 11:1 | Reserved (0) |
> > + | N-1:12 | Physical address of 4KB aligned SPPT LX-1 Table referenced by this entry |
> > + | 51:N | Reserved (0) |
> > + | 63:52 | Reserved (0) |
> > + Note: N is the physical address width supported by the processor. X is the page level
> > +
> > + Format of the SPPT L1E:
> > + | Bit | Contents |
> > + | :---- | :---------------------------------------------------------------- |
> > + | 0+2i | Write permission for i-th 128 byte sub-page region. |
> > + | 1+2i | Reserved (0). |
> > + Note: 0<=i<=31
> > +
> > +5.SPPT-induced VM exit
> > +
> > + * SPPT miss and misconfiguration induced VM exit
> > +
> > + A SPPT missing VM exit occurs when walk the SPPT, there is no SPPT
> > + misconfiguration but a paging-structure entry is not
> > + present in any of L4E/L3E/L2E entries.
> > +
> > + A SPPT misconfiguration VM exit occurs when reserved bits or unsupported values
> > + are set in SPPT entry.
> > +
> > + *NOTE* SPPT miss and SPPT misconfigurations can occur only due to an
> > + attempt to write memory with a guest physical address.
>
> Can you clarify what this means? For instance, setting an A or D bit
> in a PTE is an attempt to "write memory with a guest physical
> address," but per the SDM, it is not an operation that is eligible for
> sub-page write permissions.

Yep, should be "memory write mapped by EPT leaf entry and guarded by SPP". thanks!
>
> > + * SPP permission induced VM exit
> > + SPP sub-page permission induced violation is reported as EPT violation
> > + thesefore causes VM exit.
> /thesefore/therefore/
>
> > +
> > +6.SPPT-induced VM exit handling
> > +
> > + #define EXIT_REASON_SPP 66
> > +
> > + static int (*const kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
> > + ...
> > + [EXIT_REASON_SPP] = handle_spp,
> > + ...
> > + };
> > +
> > + New exit qualification for SPPT-induced vmexits.
> > +
> > + | Bit | Contents |
> > + | :---- | :---------------------------------------------------------------- |
> > + | 10:0 | Reserved (0). |
> > + | 11 | SPPT VM exit type. Set for SPPT Miss, cleared for SPPT Misconfig. |
> > + | 12 | NMI unblocking due to IRET |
> > + | 63:13 | Reserved (0) |
> > +
> > + In addition to the exit qualification, guest linear address and guest
> > + physical address fields will be reported.
> > +
> > + * SPPT miss and misconfiguration induced VM exit
> > + Set up SPPT entries correctly.
> > +
> > + * SPP permission induced VM exit
> > + This kind of VM exit is left to VMI tool to handle.
> > --
> > 2.17.2
> >

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