Re: [Xen-devel] [PATCH v1] x86/mm: Suppresses vm_events caused by page-walks

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 8/24/18 8:49 PM, Andrew Cooper wrote:
> On 24/08/18 15:11, Alexandru Isaila wrote:
>> diff --git a/xen/arch/x86/mm/mem_access.c b/xen/arch/x86/mm/mem_access.c
>> index 03a864156..b01194d 100644
>> --- a/xen/arch/x86/mm/mem_access.c
>> +++ b/xen/arch/x86/mm/mem_access.c
>> @@ -212,7 +212,20 @@ bool p2m_mem_access_check(paddr_t gpa, unsigned long 
>> gla,
>>           d->arch.monitor.inguest_pagefault_disabled &&
>>           npfec.kind != npfec_kind_with_gla ) /* don't send a mem_event */
>>      {
>> -        hvm_emulate_one_vm_event(EMUL_KIND_NORMAL, TRAP_invalid_op, 
>> X86_EVENT_NO_EC);
>> +        struct hvm_hw_cpu ctxt;
>> +        uint32_t pfec = PFEC_page_present;
>> +        unsigned long gfn;
>> +        uint32_t gflags;
>> +
>> +        hvm_funcs.save_cpu_ctxt(v, &ctxt);
>> +        paging_get_hostmode(v)->pte_flags(v, p2m, gla, 0, ctxt.cr3, 
>> &gflags);
>> +        if ( gflags & _PAGE_RW )
>> +            pfec |= PFEC_write_access;
>> +
>> +        if ( gflags & _PAGE_USER )
>> +            pfec |= PFEC_user_mode;
> 
> As I've tried to explain before, this is architecturally incorrect. 
> Both need to be derived from the EPT violation, because they are
> properties of instruction which caused the fault, not the mapping which
> faulted.

Right, I assume you mean starting to use eff_read, eff_write, eff_exec,
and eff_user_exec from ept_qual_t:

619 /* EPT violation qualifications definitions */
620 typedef union ept_qual {
621     unsigned long raw;
622     struct {
623         bool read:1, write:1, fetch:1,
624             eff_read:1, eff_write:1, eff_exec:1, /* eff_user_exec */:1,
625             gla_valid:1,
626             gla_fault:1; /* Valid iff gla_valid. */
627         unsigned long /* pad */:55;
628     };
629 } __transparent__ ept_qual_t;

Unfortunately, I've been told that that's not the way to go since those
fields will be relevant only on newer architectures, and we'd like to be
able to support even older ones. And, of course, the other thing is that
they're VMX / EPT specific, and we were hoping to be able to get SVM
support for free like that.

Would we be able to properly emulate just the walk here (with correct
input) in any way that would support older architectures as well? If
not, is my understanding correct that eff_user_exec would then stand for
PFEC_user_mode, and eff_write for PFEC_write_access in the logic you've
quoted?


Thanks,
Razvan

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