Re: [PATCH v4 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server.

From: George Dunlap <george.dunlap@citrix.com>
To: Jan Beulich <JBeulich@suse.com>, Yu Zhang <yu.c.zhang@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>,
	George Dunlap <george.dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Tim Deegan <tim@xen.org>,
	xen-devel@lists.xen.org, Paul Durrant <paul.durrant@citrix.com>,
	zhiyuan.lv@intel.com, Jun Nakajima <jun.nakajima@intel.com>
Subject: Re: [PATCH v4 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server.
Date: Tue, 14 Jun 2016 14:13:35 +0100	[thread overview]
Message-ID: <576002FF.3030008@citrix.com> (raw)
In-Reply-To: <575FFC7E02000078000F4BA1@prv-mh.provo.novell.com>

On 14/06/16 11:45, Jan Beulich wrote:
>> +                         struct hvm_ioreq_server *s)
>> +{
>> +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
>> +    int rc;
>> +
>> +    spin_lock(&p2m->ioreq.lock);
>> +
>> +    if ( flags == 0 )
>> +    {
>> +        rc = -EINVAL;
>> +        if ( p2m->ioreq.server != s )
>> +            goto out;
>> +
>> +        /* Unmap ioreq server from p2m type by passing flags with 0. */
>> +        p2m->ioreq.server = NULL;
>> +        p2m->ioreq.flags = 0;
>> +    }
> 
> What does "passing" refer to in the comment?

You make the map_memtype_... hypercall with "flags" set to 0.  I'm not
sure what's unclear about the sentence; how would you put it differently?

> Locking is somewhat strange here: You protect against the "set"
> counterpart altering state while you retrieve it, but you don't
> protect against the returned data becoming stale by the time
> the caller can consume it. Is that not a problem? (The most
> concerning case would seem to be a race of hvmop_set_mem_type()
> with de-registration of the type.)

How is that different than calling set_mem_type() first, and then
de-registering without first unmapping all the types?

>> +    uint32_t flags;     /* IN - types of accesses to be forwarded to the
>> +                           ioreq server. flags with 0 means to unmap the
>> +                           ioreq server */
>> +#define _HVMOP_IOREQ_MEM_ACCESS_READ 0
>> +#define HVMOP_IOREQ_MEM_ACCESS_READ \
>> +    (1u << _HVMOP_IOREQ_MEM_ACCESS_READ)
>> +
>> +#define _HVMOP_IOREQ_MEM_ACCESS_WRITE 1
>> +#define HVMOP_IOREQ_MEM_ACCESS_WRITE \
>> +    (1u << _HVMOP_IOREQ_MEM_ACCESS_WRITE)
> 
> Is there any use for these _HVMOP_* values? The more that they
> violate standard C name space rules?

I assume he's just going along with what he sees in params.h.
"Violating standard C name space rules" by having #defines which start
with a single _ seems to be a well-established policy for Xen. :-)

 -George

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