Re: [Xen-devel] [PATCH v2] arch/x86: Add registers to vm_event

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On 17.10.2018 12:49, Andrew Cooper wrote:
> On 17/10/18 10:39, Alexandru Stefan ISAILA wrote:
>> This patch adds a couple of regs to the vm_event that are used by
>> the introspection. The base, limit and ar
>> bits are compressed into a uint64_t union so as not to enlarge the
>> vm_event.
>>
>> Signed-off-by: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
>>
>> ---
>> Changes since V1:
>>      - Add helper function for packing
>>      - Change packing logic
>>      - Add sel to x86_selector_reg.
>> ---
>>   xen/arch/x86/vm_event.c       | 30 ++++++++++++++++++++++--------
>>   xen/include/public/vm_event.h | 26 ++++++++++++++++++++++----
>>   2 files changed, 44 insertions(+), 12 deletions(-)
>>
>> diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
>> index 15de43c3e6..3a29441c84 100644
>> --- a/xen/arch/x86/vm_event.c
>> +++ b/xen/arch/x86/vm_event.c
>> @@ -122,11 +122,25 @@ void vm_event_monitor_next_interrupt(struct vcpu *v)
>>       v->arch.monitor.next_interrupt_enabled = true;
>>   }
>>   
>> +static void vm_event_pack_segment_register(enum x86_segment segment,
>> +                                           struct x86_selector_reg *reg)
>> +{
>> +    struct segment_register seg;
>> +
>> +    hvm_get_segment_register(current, segment, &seg);
>> +    reg->fields.base = seg.base;
>> +    if ( seg.g )
>> +        reg->fields.limit = seg.limit >> 12;
>> +    else
>> +        reg->fields.limit = seg.limit;
> 
> I know I suggested this, but how about
> 
> reg->fields.limit = seg.g ? seg.limit >> 12 : seg.limit;
> 
> which is rather shorter, and probably easier to read.

OK, I will revise this on the next version

> 
>> +    reg->fields.ar = seg.attr;
>> +    reg->sel = seg.sel;
>> +}
>> +
>>   void vm_event_fill_regs(vm_event_request_t *req)
>>   {
>>   #ifdef CONFIG_HVM
>>       const struct cpu_user_regs *regs = guest_cpu_user_regs();
>> -    struct segment_register seg;
>>       struct hvm_hw_cpu ctxt = {};
>>       struct vcpu *curr = current;
>>   
>> @@ -170,14 +184,14 @@ void vm_event_fill_regs(vm_event_request_t *req)
>>       req->data.regs.x86.msr_star = ctxt.msr_star;
>>       req->data.regs.x86.msr_lstar = ctxt.msr_lstar;
>>   
>> -    hvm_get_segment_register(curr, x86_seg_fs, &seg);
>> -    req->data.regs.x86.fs_base = seg.base;
>> -
>> -    hvm_get_segment_register(curr, x86_seg_gs, &seg);
>> -    req->data.regs.x86.gs_base = seg.base;
>> +    vm_event_pack_segment_register(x86_seg_fs, &req->data.regs.x86.fs);
>> +    vm_event_pack_segment_register(x86_seg_gs, &req->data.regs.x86.gs);
>> +    vm_event_pack_segment_register(x86_seg_cs, &req->data.regs.x86.cs);
>> +    vm_event_pack_segment_register(x86_seg_ss, &req->data.regs.x86.ss);
>> +    vm_event_pack_segment_register(x86_seg_ds, &req->data.regs.x86.ds);
>> +    vm_event_pack_segment_register(x86_seg_es, &req->data.regs.x86.es);
>>   
>> -    hvm_get_segment_register(curr, x86_seg_cs, &seg);
>> -    req->data.regs.x86.cs_arbytes = seg.attr;
>> +    req->data.regs.x86.shadow_gs = ctxt.shadow_gs;
>>   #endif
>>   }
>>   
>> diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
>> index 36e3f4685d..705e6f8a1c 100644
>> --- a/xen/include/public/vm_event.h
>> +++ b/xen/include/public/vm_event.h
>> @@ -29,7 +29,7 @@
>>   
>>   #include "xen.h"
>>   
>> -#define VM_EVENT_INTERFACE_VERSION 0x00000003
>> +#define VM_EVENT_INTERFACE_VERSION 0x00000004
>>   
>>   #if defined(__XEN__) || defined(__XEN_TOOLS__)
>>   
>> @@ -157,6 +157,20 @@
>>   #define VM_EVENT_X86_CR4    2
>>   #define VM_EVENT_X86_XCR0   3
>>   
>> +struct __attribute__((__packed__)) x86_selector_reg {
> 
> This is a poor choice of name, because you are mixing two different
> pieces of information.
> 
> sel is a selector value, whereas the union is a segment descriptor
> (architecturally speaking).

I will take sel out of the structure

> 
>> +    uint16_t sel;
>> +    union
>> +    {
>> +        uint64_t bytes;
>> +        struct
>> +        {
>> +            uint64_t base   :    32;
> 
> Better known as... ?

Sorry I don't follow here

> 
>> +            uint64_t limit  :    20;
>> +            uint64_t ar     :    12;
>> +        } fields;
>> +    };
>> +};
>> +
>>   /*
>>    * Using custom vCPU structs (i.e. not hvm_hw_cpu) for both x86 and ARM
>>    * so as to not fill the vm_event ring buffer too quickly.
>> @@ -191,9 +205,13 @@ struct vm_event_regs_x86 {
>>       uint64_t msr_efer;
>>       uint64_t msr_star;
>>       uint64_t msr_lstar;
>> -    uint64_t fs_base;
>> -    uint64_t gs_base;
> 
> What's happened with fs_base and gs_base?  You've replaced them with
> uint32_t's in x86_selector_reg, but they are 64bit fields in Long mode.
> 
> ~Andrew

I will put fs_base and gs_base back in with 64bit

Alex
> 
>> -    uint32_t cs_arbytes;
>> +    struct x86_selector_reg cs;
>> +    struct x86_selector_reg ss;
>> +    struct x86_selector_reg ds;
>> +    struct x86_selector_reg es;
>> +    struct x86_selector_reg fs;
>> +    struct x86_selector_reg gs;
>> +    uint64_t shadow_gs;
>>       uint32_t _pad;
>>   };
>>   
> 
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