Problems calling HVMOP_flush_tlbs

Problems calling HVMOP_flush_tlbs

[Antony Saba]

Hello xen-devel,

I am using xen 4.2.1 and have tried the following with kernel 3.0.0 and 
3.2.1.

I am trying to invoke the HVMOP_flush_tlbs hypercall but have been 
unsuccessful.  I am basing my call on the functions in in xc_misc.c, 
trying to guess what is meant by "@arg must be null" in the comment 
where HVMOP_flush_tlbs is defined.  What is the correct way to invoke 
this hypercall?

If I call it like this, I receive an invalid parameter (EINVAL) error:
     struct privcmd_hypercall
     {
         uint64_t op;
         uint64_t arg[5];
     } hypercall;
     hypercall.op     = __HYPERVISOR_hvm_op;
     hypercall.arg[0] = HVMOP_flush_tlbs;
     hypercall.arg[1] = 0;
     ret = do_xen_hypercall(xch, (void*)&hypercall);


If I call it like this, I get function not implemented (ENOSYS), where 3 
is a valid domain id:
     hypercall.op     = __HYPERVISOR_hvm_op;
     hypercall.arg[0] = HVMOP_flush_tlbs;
     hypercall.arg[1] = 3;  /* 3 is the domain whose tlbs should be 
flushed */
     hypercall.arg[2] = 0;

I have also tried this, mimicking the arg struct for the other HVMOP 
calls, which also returns ENOSYS:
    typedef struct _arg_t {
         domid_t domid;
         uint64_t ptr;
    } arg_t;
    struct privcmd_hypercall {
         uint64_t op;
         uint64_t arg[5];
     } hypercall;
     DECLARE_HYPERCALL_BUFFER(arg_t, arg);
     arg = xc_hypercall_buffer_alloc(xch, arg, sizeof(*arg));

     hypercall.op     = __HYPERVISOR_hvm_op;
     hypercall.arg[0] = HVMOP_flush_tlbs;
     hypercall.arg[1] = HYPERCALL_BUFFER_AS_ARG(arg);
     hypercall.arg[2]= 0;
     arg->ptr = 0;
     arg->domid = 0; /* or a valid dom id, the results are the same */


Any help is appreciated.

-Tony

Re: Problems calling HVMOP_flush_tlbs

[Tim Deegan]

Hi, 

At 00:50 +0000 on 17 Jan (1326761401), Antony Saba wrote:
> Hello xen-devel,
> 
> I am using xen 4.2.1 and have tried the following with kernel 3.0.0 and 
> 3.2.1.
> 
> I am trying to invoke the HVMOP_flush_tlbs hypercall but have been 
> unsuccessful.  I am basing my call on the functions in in xc_misc.c, 
> trying to guess what is meant by "@arg must be null" in the comment 
> where HVMOP_flush_tlbs is defined.  What is the correct way to invoke 
> this hypercall?
> 
> If I call it like this, I receive an invalid parameter (EINVAL) error:
>      struct privcmd_hypercall
>      {
>          uint64_t op;
>          uint64_t arg[5];
>      } hypercall;
>      hypercall.op     = __HYPERVISOR_hvm_op;
>      hypercall.arg[0] = HVMOP_flush_tlbs;
>      hypercall.arg[1] = 0;
>      ret = do_xen_hypercall(xch, (void*)&hypercall);
> 
> 
> If I call it like this, I get function not implemented (ENOSYS), where 3 
> is a valid domain id:
>      hypercall.op     = __HYPERVISOR_hvm_op;
>      hypercall.arg[0] = HVMOP_flush_tlbs;
>      hypercall.arg[1] = 3;  /* 3 is the domain whose tlbs should be 
> flushed */
>      hypercall.arg[2] = 0;

Did you look at the code of Xen?  A quick grep finds this:

    case HVMOP_flush_tlbs:
        rc = guest_handle_is_null(arg) ? hvmop_flush_tlb_all() : -ENOSYS;
        break;

So that's what "@arg must ne null" means: if you provide an argument,
you get ENOSYS.  And just inside hvmop_flush_tlb_all() is this:

    if ( !is_hvm_domain(d) )
        return -EINVAL;

So if you call from a non-HVM domain, you get EINVAL.  That matches what
you're seeing. 

I think the important detail is that HVMOP_flush_tlbs doesn't take a target
domain - it always flushes the _caller's_ TLBs.  I guess you want to
flush the TLBs of another domain for some reason?  AFAICS the version in
mainline Xen has never taken an argument so perhaps you could add one
(making sure to keep backward compatibility, of course).

Cheers,

Tim.

Re: Problems calling HVMOP_flush_tlbs

[Antony Saba]

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On 01/17/2012 04:08 AM, Tim Deegan wrote

I think the important detail is that HVMOP_flush_tlbs doesn't take a target
domain - it always flushes the _caller's_ TLBs.  I guess you want to
flush the TLBs of another domain for some reason?  AFAICS the version in
mainline Xen has never taken an argument so perhaps you could add one
(making sure to keep backward compatibility, of course).



Thanks, Tim.

Yes, I am trying flush a guests TLBs from dom0.  I am using mem_events, and are seeing events that are not caught all the time.  The difference appears to be that the guest does a mov cr3 in the case where they are caught.

I was hoping it was a simple matter of flushing the guest TLBs somehow.

-Tony


--
Antony Saba, antony.saba@mandiant.com<mailto:antony.saba@mandiant.com>

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