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Re: [Xen-devel] [PATCH v6 1/4] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server.
On 9/21/2016 9:04 PM, George Dunlap wrote:
On Fri, Sep 9, 2016 at 6:51 AM, Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
On 9/2/2016 6:47 PM, Yu Zhang wrote:
A new HVMOP - HVMOP_map_mem_type_to_ioreq_server, is added to
let one ioreq server claim/disclaim its responsibility for the
handling of guest pages with p2m type p2m_ioreq_server. Users
of this HVMOP can specify which kind of operation is supposed
to be emulated in a parameter named flags. Currently, this HVMOP
only support the emulation of write operations. And it can be
further extended to support the emulation of read ones if an
ioreq server has such requirement in the future.
For now, we only support one ioreq server for this p2m type, so
once an ioreq server has claimed its ownership, subsequent calls
of the HVMOP_map_mem_type_to_ioreq_server will fail. Users can also
disclaim the ownership of guest ram pages with p2m_ioreq_server, by
triggering this new HVMOP, with ioreq server id set to the current
owner's and flags parameter set to 0.
Note both HVMOP_map_mem_type_to_ioreq_server and p2m_ioreq_server
are only supported for HVMs with HAP enabled.
Also note that only after one ioreq server claims its ownership
of p2m_ioreq_server, will the p2m type change to p2m_ioreq_server
be allowed.
Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
Acked-by: Tim Deegan <tim@xxxxxxx>
---
Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
changes in v6:
- Clarify logic in hvmemul_do_io().
- Use recursive lock for ioreq server lock.
- Remove debug print when mapping ioreq server.
- Clarify code in ept_p2m_type_to_flags() for consistency.
- Remove definition of P2M_IOREQ_HANDLE_WRITE_ACCESS.
- Add comments for HVMMEM_ioreq_server to note only changes
to/from HVMMEM_ram_rw are permitted.
- Add domain_pause/unpause() in hvm_map_mem_type_to_ioreq_server()
to avoid the race condition when a vm exit happens on a write-
protected page, just to find the ioreq server has been unmapped
already.
- Introduce a seperate patch to delay the release of p2m
lock to avoid the race condition.
- Introduce a seperate patch to handle the read-modify-write
operations on a write protected page.
Why do we need to do this? Won't the default case just DTRT if it finds
that the ioreq server has been unmapped?
Well, patch 4 will either mark the remaining p2m_ioreq_server entries as
"recal" or
reset to p2m_ram_rw directly. So my understanding is that we do not wish to
see a ept violation due to a p2m_ioreq_server access after the ioreq server
is unmapped.
Yet without this domain_pause/unpause() pair, VM accesses may trigger an ept
violation
during the hvmop hypercall(hvm_map_mem_type_to_ioreq_server), just to find
the ioreq
server is NULL. Then we would have to provide handlers which just do the
copy to/from
actions for the VM. This seems awkward to me.
So the race you're worried about is this:
1. Guest fault happens
2. ioreq server calls map_mem_type_to_ioreq_server, unhooking
3. guest finds no ioreq server present
I think in that case the easiest thing to do would be to simply assume
there was a race and re-execute the instruction. Is that not possible
for some reason?
-George
Thanks for your reply, George. :)
Two reasons I'd like to use the domain_pause/unpause() to avoid the race
condition:
1> Like my previous explanation, in the read-modify-write scenario, the
ioreq server will
be NULL for the read emulation. But in such case, hypervisor will not
discard this trap, instead
it is supposed to do the copy work for the read access. So it would be
difficult for hypervisor
to decide if the ioreq server was detached due to a race condition, or
if the ioreq server should
be a NULL because we are emulating a read operation first for a
read-modify-write instruction.
2> I also realized it can avoid a deadlock possibility -
a> dom0 triggers map_mem_type_to_ioreq_server, which spin locks the
ioreq_server.lock,
and before routine hvm_map_mem_type_to_ioreq_server() returns,
b> The HVM triggers an ept violation, and p2m lock is held by
hvm_hap_nested_page_fault();
c> hypervisor continues to the I/O handler, which will need the
ioreq_server.lock to select
an ioreq server, which is being held by the hypercall handler
side;
d> likewise, the map_mem_type_to_ioreq_server will meet problem
when trying to get the
p2m lock when it calls p2m_change_entry_type_global().
With a domain_pause/unpause(), we could avoid the ept violation
during this period(which I
believe won't be long because this pair only covers the
p2m_change_entry_type_global() call, not
the p2m_finish_type_change() call).
Thanks
Yu
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