Re: [Xen-devel] [PATCH 1/2] x86/mem_event: Deliver gla fault EPT violation information

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/08/14 18:14, Tamas Lengyel wrote:

According to the Intel manual the gla_fault bit is only set if bit 7 (gla_valid) is 1 and it is reserved if bit 7 is 0 (cleared to 0). So the combination of gla_valid = 0 and gla_fault = 0 is safe to use as a fallback. Furthermore, there is no mem_access support for AMD NPT so at this point this is just a placeholder in the svm code.

That is entirely my point.  You are putting Intel-isms in a common interface which AMD currently cant use, and might cause issues for someone trying to reintroduce feature parity.

I think at a minimum, I think you need a gla_fault_info_valid bit unless you can guarentee that AMD NPT will never gain the ability to report a valid gla. (I don't know whether it can or not.  Looking at the code, it would appear that it can't currently, but I certainly not bet against it ever gaining this ability.)

~Andrew

On Thu, Aug 7, 2014 at 6:50 PM, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:

On 07/08/14 17:43, Tamas K Lengyel wrote:
> On Intel EPT the exit qualification generated by a violation also includes a bit (EPT_GLA_FAULT) which describes the following information:
> Set if the access causing the EPT violation is to a guest-physical address that is the translation of a linear address. Clear if the access causing the EPT violation is to a paging-structure entry as part of a page walk or the update of an accessed or dirty bit.
>
> For more information see Table 27-7 in the Intel SDM.
>
> This patch extends the mem_event system to deliver this extra information, which could be useful for determining the cause of a violation.
>
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
> ---
>  xen/arch/x86/hvm/hvm.c         | 6 ++++--
>  xen/arch/x86/hvm/svm/svm.c     | 2 +-
>  xen/arch/x86/hvm/vmx/vmx.c     | 4 +++-
>  xen/arch/x86/mm/p2m.c          | 7 ++++---
>  xen/include/asm-x86/hvm/hvm.h  | 4 +++-
>  xen/include/asm-x86/p2m.h      | 6 +++---
>  xen/include/public/mem_event.h | 3 ++-
>  7 files changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index e834406..b09a905 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -2724,6 +2724,7 @@ void hvm_inject_page_fault(int errcode, unsigned long cr2)
>
>  int hvm_hap_nested_page_fault(paddr_t gpa,
>                                bool_t gla_valid,
> +                              bool_t gla_fault,
>                                unsigned long gla,
>                                bool_t access_r,
>                                bool_t access_w,
> @@ -2832,8 +2833,9 @@ int hvm_hap_nested_page_fault(paddr_t gpa,
>
>          if ( violation )
>          {
> -            if ( p2m_mem_access_check(gpa, gla_valid, gla, access_r,
> -                                        access_w, access_x, &req_ptr) )
> +            if ( p2m_mem_access_check(gpa, gla_valid, gla_fault, gla,
> +                                        access_r, access_w, access_x,
> +                                        &req_ptr) )
>              {
>                  fall_through = 1;
>              } else {
> diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
> index 76616ac..1dbb12f 100644
> --- a/xen/arch/x86/hvm/svm/svm.c
> +++ b/xen/arch/x86/hvm/svm/svm.c
> @@ -1403,7 +1403,7 @@ static void svm_do_nested_pgfault(struct vcpu *v,
>      p2m_access_t p2ma;
>      struct p2m_domain *p2m = NULL;
>
> -    ret = hvm_hap_nested_page_fault(gpa, 0, ~0ul,
> +    ret = hvm_hap_nested_page_fault(gpa, 0, 0, ~0ul,

How do you expect this to work?  You don't know whether it was a linear
fault or a paging-structure fault for the NPT case, so presenting it is
a paging-structure fault is certainly wrong.

~Andrew

>                                      1, /* All NPFs count as reads */
>                                      npfec & PFEC_write_access,
>                                      npfec & PFEC_insn_fetch);
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index 2caa04a..0bd77b3 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -2375,7 +2375,9 @@ static void ept_handle_violation(unsigned long qualification, paddr_t gpa)
>      else
>          gla = ~0ull;
>      ret = hvm_hap_nested_page_fault(gpa,
> -                                    !!(qualification & EPT_GLA_VALID), gla,
> +                                    !!(qualification & EPT_GLA_VALID),
> +                                    !!(qualification & EPT_GLA_FAULT),
> +                                    gla,
>                                      !!(qualification & EPT_READ_VIOLATION),
>                                      !!(qualification & EPT_WRITE_VIOLATION),
>                                      !!(qualification & EPT_EXEC_VIOLATION));
> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> index bca9f0f..303dc2b 100644
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -1323,9 +1323,9 @@ void p2m_mem_paging_resume(struct domain *d)
>      }
>  }
>
> -bool_t p2m_mem_access_check(paddr_t gpa, bool_t gla_valid, unsigned long gla,
> -                          bool_t access_r, bool_t access_w, bool_t access_x,
> -                          mem_event_request_t **req_ptr)
> +bool_t p2m_mem_access_check(paddr_t gpa, bool_t gla_valid, bool_t gla_fault,
> +                          unsigned long gla, bool_t access_r, bool_t access_w,
> +                          bool_t access_x, mem_event_request_t **req_ptr)
>  {
>      struct vcpu *v = current;
>      unsigned long gfn = gpa >> PAGE_SHIFT;
> @@ -1404,6 +1404,7 @@ bool_t p2m_mem_access_check(paddr_t gpa, bool_t gla_valid, unsigned long gla,
>          req->gfn = gfn;
>          req->offset = gpa & ((1 << PAGE_SHIFT) - 1);
>          req->gla_valid = gla_valid;
> +        req->gla_fault = gla_fault;
>          req->gla = gla;
>          req->access_r = access_r;
>          req->access_w = access_w;
> diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
> index 0ebd478..38562ff 100644
> --- a/xen/include/asm-x86/hvm/hvm.h
> +++ b/xen/include/asm-x86/hvm/hvm.h
> @@ -456,7 +456,9 @@ static inline void hvm_invalidate_regs_fields(struct cpu_user_regs *regs)
>  }
>
>  int hvm_hap_nested_page_fault(paddr_t gpa,
> -                              bool_t gla_valid, unsigned long gla,
> +                              bool_t gla_valid,
> +                              bool_t gla_fault,
> +                              unsigned long gla,
>                                bool_t access_r,
>                                bool_t access_w,
>                                bool_t access_x);
> diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
> index 0ddbadb..8616fda 100644
> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -597,9 +597,9 @@ void p2m_mem_paging_resume(struct domain *d);
>   * been promoted with no underlying vcpu pause. If the req_ptr has been populated,
>   * then the caller must put the event in the ring (once having released get_gfn*
>   * locks -- caller must also xfree the request. */
> -bool_t p2m_mem_access_check(paddr_t gpa, bool_t gla_valid, unsigned long gla,
> -                          bool_t access_r, bool_t access_w, bool_t access_x,
> -                          mem_event_request_t **req_ptr);
> +bool_t p2m_mem_access_check(paddr_t gpa, bool_t gla_valid, bool_t gla_fault,
> +                          unsigned long gla, bool_t access_r, bool_t access_w,
> +                          bool_t access_x, mem_event_request_t **req_ptr);
>  /* Resumes the running of the VCPU, restarting the last instruction */
>  void p2m_mem_access_resume(struct domain *d);
>
> diff --git a/xen/include/public/mem_event.h b/xen/include/public/mem_event.h
> index 3831b41..5785ff9 100644
> --- a/xen/include/public/mem_event.h
> +++ b/xen/include/public/mem_event.h
> @@ -62,7 +62,8 @@ typedef struct mem_event_st {
>      uint16_t access_w:1;
>      uint16_t access_x:1;
>      uint16_t gla_valid:1;
> -    uint16_t available:12;
> +    uint16_t gla_fault:1;
> +    uint16_t available:11;
>
>      uint16_t reason;
>  } mem_event_request_t, mem_event_response_t;

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