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SubjectRe: [PATCH v5 1/9] Documentation: Introduce EPT based Subpage Protection
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/
> +
> + 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.

> +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?
> + 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.

> + * 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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