From: "Jan Beulich" <JBeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Wei Liu <wei.liu2@citrix.com>,
suravee.suthikulpanit@amd.com, Eddie Dong <eddie.dong@intel.com>,
Aravind Gopalakrishnan <aravind.gopalakrishnan@amd.com>,
Jun Nakajima <jun.nakajima@intel.com>,
xen-devel <xen-devel@lists.xenproject.org>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Keir Fraser <keir@xen.org>
Subject: Re: [PATCH] x86/HVM: honor p2m_ram_ro in hvm_map_guest_frame_rw()
Date: Fri, 24 Jul 2015 06:33:17 -0600 [thread overview]
Message-ID: <55B24CAD020000780009528C@prv-mh.provo.novell.com> (raw)
In-Reply-To: <55B22964.2030701@citrix.com>
>>> On 24.07.15 at 14:02, <andrew.cooper3@citrix.com> wrote:
> On 24/07/15 10:41, Jan Beulich wrote:
>> ... and its callers.
>>
>> While all non-nested users are made fully honor the semantics of that
>> type, doing so in the nested case seemed insane (if doable at all,
>> considering VMCS shadowing), and hence there the respective operations
>> are simply made fail.
>
> Sorry, but I can't parse this sentence. Surely in the nested case, it
> is the host p2m type which is relevant to whether a mapping should be
> forced read only?
No, what I mean to say is
- callers outside of nested-HVM code properly obey the write-ignore
semantics
- callers inside nested-HVM code would be too cumbersome (and
maybe impossible) to fix, and hence they're being made return
failure to their callers.
>> Beyond that log-dirty handling in _hvm_map_guest_frame() looks bogus
>> too: What if a XEN_DOMCTL_SHADOW_OP_* gets issued and acted upon
>> between the setting of the dirty flag and the actual write happening?
>> I.e. shouldn't the flag instead be set in hvm_unmap_guest_frame()?
>
> It does indeed. (Ideally the dirty bit should probably be held high for
> the duration that a mapping exists, but that is absolutely infeasible to
> do).
I don't see this being too difficult, the more that for transient
mappings it doesn't really matter (if there's a race, then setting
the flag after the write(s) is good enough). For permanent
mappings I can't see why we wouldn't be able to add a (short)
linked list of pages paging_log_dirty_op() should always set the
dirty flags for.
>> @@ -3797,6 +3805,7 @@ static int hvm_load_segment_selector(
>> break;
>> }
>> } while ( !(desc.b & 0x100) && /* Ensure Accessed flag is set */
>> + writable && /* except if we are to discard writes */
>> (cmpxchg(&pdesc->b, desc.b, desc.b | 0x100) != desc.b) );
>
> I can't recall where I read it in the manual, but I believe it is a
> faultable error to load a descriptor from RO memory if the accessed bit
> is not already set. This was to prevent a processor livelock when
> running with gdtr pointing into ROM (which was a considered usecase).
I don't see why a processor would live-lock in such a case. It can do
the write, and ignore whether it actually too effect. I don't see why
it would e.g. spin until it sees the flag set. (Note that a cmpxchg()
like loop alone wouldn't have that problem, i.e. for a live lock to occur
there would still need to be an outer loop doing the checking).
But even it there was such (perhaps even model specific) behavior,
without having a pointer to where this is specified (and hence what
precise fault [and error code] to raise), I wouldn't want to go that
route here.
Jan
next prev parent reply other threads:[~2015-07-24 12:33 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-24 9:41 [PATCH] x86/HVM: honor p2m_ram_ro in hvm_map_guest_frame_rw() Jan Beulich
2015-07-24 10:26 ` Wei Liu
2015-07-24 10:37 ` Jan Beulich
2015-07-24 10:41 ` Wei Liu
2015-07-24 12:02 ` Andrew Cooper
2015-07-24 12:33 ` Jan Beulich [this message]
2015-07-27 11:09 ` Tim Deegan
2015-08-11 13:51 ` Jan Beulich
2015-08-11 14:34 ` Tim Deegan
2015-08-11 15:37 ` Jan Beulich
2015-08-11 15:45 ` Tim Deegan
2015-08-11 16:01 ` Jan Beulich
2015-07-31 1:41 ` Tian, Kevin
2015-07-31 16:06 ` Boris Ostrovsky
2015-08-11 10:32 ` Jan Beulich
2015-08-14 10:38 ` Jan Beulich
2015-08-14 13:26 ` Boris Ostrovsky
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