From: "Jan Beulich" <JBeulich@suse.com>
To: Paul Durrant <paul.durrant@citrix.com>,
Zhang Yu <yu.c.zhang@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Keir Fraser <keir@xen.org>,
George Dunlap <george.dunlap@eu.citrix.com>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Tim Deegan <tim@xen.org>,
xen-devel@lists.xen.org, zhiyuan.lv@intel.com,
Jun Nakajima <jun.nakajima@intel.com>
Subject: Re: [PATCH v2 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server
Date: Mon, 11 Apr 2016 10:31:58 -0600 [thread overview]
Message-ID: <570BDF8E02000078000E63F1@prv-mh.provo.novell.com> (raw)
In-Reply-To: <570B871E.7040703@linux.intel.com>
>>> On 11.04.16 at 13:14, <yu.c.zhang@linux.intel.com> wrote:
> On 4/9/2016 6:28 AM, Jan Beulich wrote:
>>>>> On 31.03.16 at 12:53, <yu.c.zhang@linux.intel.com> wrote:
>>> @@ -168,13 +226,72 @@ static int hvmemul_do_io(
>>> break;
>>> case X86EMUL_UNHANDLEABLE:
>>> {
>>> - struct hvm_ioreq_server *s =
>>> - hvm_select_ioreq_server(curr->domain, &p);
>>> + struct hvm_ioreq_server *s;
>>> + p2m_type_t p2mt;
>>> +
>>> + if ( is_mmio )
>>> + {
>>> + unsigned long gmfn = paddr_to_pfn(addr);
>>> +
>>> + (void) get_gfn_query_unlocked(currd, gmfn, &p2mt);
>>> +
>>> + switch ( p2mt )
>>> + {
>>> + case p2m_ioreq_server:
>>> + {
>>> + unsigned long flags;
>>> +
>>> + p2m_get_ioreq_server(currd, &flags, &s);
>>
>> As the function apparently returns no value right now, please avoid
>> the indirection on both values you're after - one of the two
>> (presumably s) can be the function's return value.
>
> Well, current implementation of p2m_get_ioreq_server() has spin_lock/
> spin_unlock surrounding the reading of flags and the s, but I believe
> we can also use the s as return value.
The use of a lock inside the function has nothing to do with how it
returns values to the caller.
>>> /* If there is no suitable backing DM, just ignore accesses */
>>> if ( !s )
>>> {
>>> - rc = hvm_process_io_intercept(&null_handler, &p);
>>> + switch ( p2mt )
>>> + {
>>> + case p2m_ioreq_server:
>>> + /*
>>> + * Race conditions may exist when access to a gfn with
>>> + * p2m_ioreq_server is intercepted by hypervisor, during
>>> + * which time p2m type of this gfn is recalculated back
>>> + * to p2m_ram_rw. mem_handler is used to handle this
>>> + * corner case.
>>> + */
>>
>> Now if there is such a race condition, the race could also be with a
>> page changing first to ram_rw and then immediately further to e.g.
>> ram_ro. See the earlier comment about assuming the page to be
>> writable.
>>
>
> Thanks, Jan. After rechecking the code, I suppose the race condition
> will not happen. In hvmemul_do_io(), get_gfn_query_unlocked() is used
> to peek the p2mt for the gfn, but get_gfn_type_access() is called inside
> hvm_hap_nested_page_fault(), and this will guarantee no p2m change shall
> occur during the emulation.
> Is this understanding correct?
Ah, yes, I think so. So the comment is misleading.
>>> +static int hvm_map_mem_type_to_ioreq_server(struct domain *d,
>>> + ioservid_t id,
>>> + hvmmem_type_t type,
>>> + uint32_t flags)
>>> +{
>>> + struct hvm_ioreq_server *s;
>>> + int rc;
>>> +
>>> + /* For now, only HVMMEM_ioreq_server is supported */
>>> + if ( type != HVMMEM_ioreq_server )
>>> + return -EINVAL;
>>> +
>>> + if ( flags & ~(HVMOP_IOREQ_MEM_ACCESS_READ |
>>> + HVMOP_IOREQ_MEM_ACCESS_WRITE) )
>>> + return -EINVAL;
>>> +
>>> + spin_lock(&d->arch.hvm_domain.ioreq_server.lock);
>>> +
>>> + rc = -ENOENT;
>>> + list_for_each_entry ( s,
>>> + &d->arch.hvm_domain.ioreq_server.list,
>>> + list_entry )
>>> + {
>>> + if ( s == d->arch.hvm_domain.default_ioreq_server )
>>> + continue;
>>> +
>>> + if ( s->id == id )
>>> + {
>>> + rc = p2m_set_ioreq_server(d, flags, s);
>>> + if ( rc == 0 )
>>> + gdprintk(XENLOG_DEBUG, "%u %s type HVMMEM_ioreq_server.\n",
>>> + s->id, (flags != 0) ? "mapped to" : "unmapped from");
>>
>> Why gdprintk()? I don't think the current domain is of much
>> interest here. What would be of interest is the subject domain.
>>
>
> s->id is not the domain_id, but id of the ioreq server.
That's understood. But gdprintk() itself logs the current domain,
which isn't as useful as the subject one.
>>> --- a/xen/arch/x86/mm/p2m-ept.c
>>> +++ b/xen/arch/x86/mm/p2m-ept.c
>>> @@ -132,6 +132,19 @@ static void ept_p2m_type_to_flags(struct p2m_domain
>>> *p2m, ept_entry_t *entry,
>>> entry->r = entry->w = entry->x = 1;
>>> entry->a = entry->d = !!cpu_has_vmx_ept_ad;
>>> break;
>>> + case p2m_ioreq_server:
>>> + entry->r = !(p2m->ioreq.flags & P2M_IOREQ_HANDLE_READ_ACCESS);
>>> + /*
>>> + * write access right is disabled when entry->r is 0, but whether
>>> + * write accesses are emulated by hypervisor or forwarded to an
>>> + * ioreq server depends on the setting of p2m->ioreq.flags.
>>> + */
>>> + entry->w = (entry->r &&
>>> + !(p2m->ioreq.flags & P2M_IOREQ_HANDLE_WRITE_ACCESS));
>>> + entry->x = entry->r;
>>
>> Why would we want to allow instruction execution from such pages?
>> And with all three bits now possibly being clear, aren't we risking the
>> entries to be mis-treated as not-present ones?
>>
>
> Hah. You got me. Thanks! :)
> Now I realized it would be difficult if we wanna to emulate the read
> operations for HVM. According to Intel mannual, entry->r is to be
> cleared, so should entry->w if we do not want ept misconfig. And
> with both read and write permissions being forbidden, entry->x can be
> set only on processors with EXECUTE_ONLY capability.
> To avoid any entry to be mis-treated as not-present. We have several
> solutions:
> a> do not support the read emulation for now - we have no such usage
> case;
> b> add the check of p2m_t against p2m_ioreq_server in is_epte_present -
> a bit weird to me.
> Which one do you prefer? or any other suggestions?
That question would also need to be asked to others who had
suggested supporting both. I'd be fine with a, but I also don't view
b as too awkward.
>>> + /*
>>> + * Each time we map/unmap an ioreq server to/from p2m_ioreq_server,
>>> + * we mark the p2m table to be recalculated, so that gfns which were
>>> + * previously marked with p2m_ioreq_server can be resynced.
>>> + */
>>> + p2m_change_entry_type_global(d, p2m_ioreq_server, p2m_ram_rw);
>>
>> What does "resynced" here mean? I.e. I can see why this is wanted
>> when unmapping a server, but when mapping a server there shouldn't
>> be any such pages in the first place.
>>
>
> There shouldn't be. But if there is(misbehavior from the device model
> side), it can be recalculated back to p2m_ram_rw(which is not quite
> necessary as the unmapping case).
DM misbehavior should not result in such a problem - the hypervisor
should refuse any bad requests.
Jan
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next prev parent reply other threads:[~2016-04-11 16:31 UTC|newest]
Thread overview: 82+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-31 10:53 [PATCH v2 0/3] x86/ioreq server: introduce HVMMEM_ioreq_server mem type Yu Zhang
2016-03-31 10:53 ` [PATCH v2 1/3] x86/ioreq server: Add new functions to get/set memory types Yu Zhang
2016-04-05 13:57 ` George Dunlap
2016-04-05 14:08 ` George Dunlap
2016-04-08 13:25 ` Andrew Cooper
2016-03-31 10:53 ` [PATCH v2 2/3] x86/ioreq server: Rename p2m_mmio_write_dm to p2m_ioreq_server Yu Zhang
2016-04-05 14:38 ` George Dunlap
2016-04-08 13:26 ` Andrew Cooper
2016-04-08 21:48 ` Jan Beulich
2016-04-18 8:41 ` Paul Durrant
2016-04-18 9:10 ` George Dunlap
2016-04-18 9:14 ` Wei Liu
2016-04-18 9:45 ` Paul Durrant
2016-04-18 16:40 ` Jan Beulich
2016-04-18 16:45 ` Paul Durrant
2016-04-18 16:47 ` Jan Beulich
2016-04-18 16:58 ` Paul Durrant
2016-04-19 11:02 ` Yu, Zhang
2016-04-19 11:15 ` Paul Durrant
2016-04-19 11:38 ` Yu, Zhang
2016-04-19 11:50 ` Paul Durrant
2016-04-19 16:51 ` Jan Beulich
2016-04-20 14:59 ` Wei Liu
2016-04-20 15:02 ` George Dunlap
2016-04-20 16:30 ` George Dunlap
2016-04-20 16:52 ` Jan Beulich
2016-04-20 16:58 ` Paul Durrant
2016-04-20 17:06 ` George Dunlap
2016-04-20 17:09 ` Paul Durrant
2016-04-21 12:24 ` Yu, Zhang
2016-04-21 13:31 ` Paul Durrant
2016-04-21 13:48 ` Yu, Zhang
2016-04-21 13:56 ` Paul Durrant
2016-04-21 14:09 ` George Dunlap
2016-04-20 17:08 ` George Dunlap
2016-04-21 12:04 ` Yu, Zhang
2016-03-31 10:53 ` [PATCH v2 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server Yu Zhang
[not found] ` <20160404082556.GC28633@deinos.phlegethon.org>
2016-04-05 6:01 ` Yu, Zhang
2016-04-06 17:13 ` George Dunlap
2016-04-07 7:01 ` Yu, Zhang
[not found] ` <CAFLBxZbLp2zWzCzQTaJNWbanQSmTJ57ZyTh0qaD-+YUn8o8pyQ@mail.gmail.com>
2016-04-08 10:39 ` George Dunlap
[not found] ` <5707839F.9060803@linux.intel.com>
2016-04-08 11:01 ` George Dunlap
2016-04-11 11:15 ` Yu, Zhang
2016-04-14 10:45 ` Yu, Zhang
2016-04-18 15:57 ` Paul Durrant
2016-04-19 9:11 ` Yu, Zhang
2016-04-19 9:21 ` Paul Durrant
2016-04-19 9:44 ` Yu, Zhang
2016-04-19 10:05 ` Paul Durrant
2016-04-19 11:17 ` Yu, Zhang
2016-04-19 11:47 ` Paul Durrant
2016-04-19 11:59 ` Yu, Zhang
2016-04-20 14:50 ` George Dunlap
2016-04-20 14:57 ` Paul Durrant
2016-04-20 15:37 ` George Dunlap
2016-04-20 16:30 ` Paul Durrant
2016-04-20 16:58 ` George Dunlap
2016-04-21 13:28 ` Yu, Zhang
2016-04-21 13:21 ` Yu, Zhang
2016-04-22 11:27 ` Wei Liu
2016-04-22 11:30 ` George Dunlap
2016-04-19 4:37 ` Tian, Kevin
2016-04-19 9:21 ` Yu, Zhang
2016-04-08 13:33 ` Andrew Cooper
2016-04-11 11:14 ` Yu, Zhang
2016-04-11 12:20 ` Andrew Cooper
2016-04-11 16:25 ` Jan Beulich
2016-04-08 22:28 ` Jan Beulich
2016-04-11 11:14 ` Yu, Zhang
2016-04-11 16:31 ` Jan Beulich [this message]
2016-04-12 9:37 ` Yu, Zhang
2016-04-12 15:08 ` Jan Beulich
2016-04-14 9:56 ` Yu, Zhang
2016-04-19 4:50 ` Tian, Kevin
2016-04-19 8:46 ` Paul Durrant
2016-04-19 9:27 ` Yu, Zhang
2016-04-19 9:40 ` Paul Durrant
2016-04-19 9:49 ` Yu, Zhang
2016-04-19 10:01 ` Paul Durrant
2016-04-19 9:54 ` Yu, Zhang
2016-04-19 9:15 ` Yu, Zhang
2016-04-19 9:23 ` Paul Durrant
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