Re: [Xen-devel] Problems calling HVMOP_flush_tlbs

[Tim Deegan <tim@xxxxxxx>]

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.


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