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From: Julien Grall <julien@xen.org>
To: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	Oleksandr Tyshchenko <olekstysh@gmail.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Oleksandr Tyshchenko <Oleksandr_Tyshchenko@epam.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien.grall@arm.com>
Subject: Re: [PATCH V3 15/23] xen/arm: Stick around in leave_hypervisor_to_guest until I/O has completed
Date: Tue, 1 Dec 2020 12:46:56 +0000	[thread overview]
Message-ID: <c3cfe831-0f78-8e57-d9d4-802be7ce283e@xen.org> (raw)
In-Reply-To: <87czzu7emb.fsf@epam.com>

Hi Volodymyr,

On 30/11/2020 20:51, Volodymyr Babchuk wrote:
> Oleksandr Tyshchenko writes:
> 
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>
>> This patch adds proper handling of return value of
>> vcpu_ioreq_handle_completion() which involves using a loop
>> in leave_hypervisor_to_guest().
>>
>> The reason to use an unbounded loop here is the fact that vCPU
>> shouldn't continue until an I/O has completed. In Xen case, if an I/O
>> never completes then it most likely means that something went horribly
>> wrong with the Device Emulator. And it is most likely not safe to
>> continue. So letting the vCPU to spin forever if I/O never completes
>> is a safer action than letting it continue and leaving the guest in
>> unclear state and is the best what we can do for now.
>>
>> This wouldn't be an issue for Xen as do_softirq() would be called at
>> every loop. In case of failure, the guest will crash and the vCPU
>> will be unscheduled.
>>
> 
> Why you don't block vcpu there and unblock it when response is ready?

The vCPU will already block while waiting for the event channel. See the 
call wait_for_event_channel() in the ioreq code. However, you can still 
receive spurious unblock (e.g. the event channel notificaiton is 
received before the I/O is unhandled). So you have to loop around and 
check if there are more work to do.

> If
> I got it right, "client" vcpu will spin in the loop, eating own
> scheduling budget with no useful work done.

You can't really do much about that if you have a rogue/buggy device model.

> In the worst case, it will
> prevent "server" vcpu from running.

How so? Xen will raise the schedule softirq if the I/O is handled. You 
would have a pretty buggy system if your "client" vCPU is considered to 
be a much higher priority than your "server" vCPU.

Cheers,

-- 
Julien Grall


  reply	other threads:[~2020-12-01 12:47 UTC|newest]

Thread overview: 127+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-30 10:31 Oleksandr Tyshchenko
2020-11-30 10:31 ` [PATCH V3 01/23] x86/ioreq: Prepare IOREQ feature for making it common Oleksandr Tyshchenko
2020-12-01 11:03   ` Alex Bennée
2020-12-01 18:53     ` Oleksandr
2020-12-01 19:36       ` Alex Bennée
2020-12-02  8:00       ` Jan Beulich
2020-12-02 11:19         ` Oleksandr
2020-12-07 11:13   ` Jan Beulich
2020-12-07 15:27     ` Oleksandr
2020-12-07 16:29       ` Jan Beulich
2020-12-07 17:21         ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 02/23] x86/ioreq: Add IOREQ_STATUS_* #define-s and update code for moving Oleksandr Tyshchenko
2020-12-01 11:07   ` Alex Bennée
2020-12-07 11:19   ` Jan Beulich
2020-12-07 15:37     ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 03/23] x86/ioreq: Provide out-of-line wrapper for the handle_mmio() Oleksandr Tyshchenko
2020-12-07 11:27   ` Jan Beulich
2020-12-07 15:39     ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 04/23] xen/ioreq: Make x86's IOREQ feature common Oleksandr Tyshchenko
2020-12-07 11:41   ` Jan Beulich
2020-12-07 19:43     ` Oleksandr
2020-12-08  9:21       ` Jan Beulich
2020-12-08 13:56         ` Oleksandr
2020-12-08 15:02           ` Jan Beulich
2020-12-08 17:24             ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 05/23] xen/ioreq: Make x86's hvm_ioreq_needs_completion() common Oleksandr Tyshchenko
2020-12-07 11:47   ` Jan Beulich
2020-11-30 10:31 ` [PATCH V3 06/23] xen/ioreq: Make x86's hvm_mmio_first(last)_byte() common Oleksandr Tyshchenko
2020-12-07 11:48   ` Jan Beulich
2020-11-30 10:31 ` [PATCH V3 07/23] xen/ioreq: Make x86's hvm_ioreq_(page/vcpu/server) structs common Oleksandr Tyshchenko
2020-12-07 11:54   ` Jan Beulich
2020-11-30 10:31 ` [PATCH V3 08/23] xen/ioreq: Move x86's ioreq_server to struct domain Oleksandr Tyshchenko
2020-12-07 12:04   ` Jan Beulich
2020-12-07 12:12     ` Paul Durrant
2020-12-07 19:52     ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 09/23] xen/dm: Make x86's DM feature common Oleksandr Tyshchenko
2020-12-07 12:08   ` Jan Beulich
2020-12-07 20:23     ` Oleksandr
2020-12-08  9:30       ` Jan Beulich
2020-12-08 14:54         ` Oleksandr
2021-01-07 14:38           ` Oleksandr
2021-01-07 15:01             ` Jan Beulich
2021-01-07 16:49               ` Oleksandr
2021-01-12 22:23                 ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 10/23] xen/mm: Make x86's XENMEM_resource_ioreq_server handling common Oleksandr Tyshchenko
2020-12-07 11:35   ` Jan Beulich
2020-12-07 12:11     ` Jan Beulich
2020-12-07 21:06       ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 11/23] xen/ioreq: Move x86's io_completion/io_req fields to struct vcpu Oleksandr Tyshchenko
2020-12-07 12:32   ` Jan Beulich
2020-12-07 20:59     ` Oleksandr
2020-12-08  7:52       ` Paul Durrant
2020-12-08  9:35         ` Jan Beulich
2020-12-08 18:21         ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 12/23] xen/ioreq: Remove "hvm" prefixes from involved function names Oleksandr Tyshchenko
2020-12-07 12:45   ` Jan Beulich
2020-12-07 20:28     ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 13/23] xen/ioreq: Use guest_cmpxchg64() instead of cmpxchg() Oleksandr Tyshchenko
2020-12-09 21:32   ` Stefano Stabellini
2020-12-09 22:34     ` Oleksandr
2020-12-10  2:30       ` Stefano Stabellini
2020-11-30 10:31 ` [PATCH V3 14/23] arm/ioreq: Introduce arch specific bits for IOREQ/DM features Oleksandr Tyshchenko
2020-12-09 22:04   ` Stefano Stabellini
2020-12-09 22:49     ` Oleksandr
2020-12-10  2:30       ` Stefano Stabellini
2020-11-30 10:31 ` [PATCH V3 15/23] xen/arm: Stick around in leave_hypervisor_to_guest until I/O has completed Oleksandr Tyshchenko
2020-11-30 20:51   ` Volodymyr Babchuk
2020-12-01 12:46     ` Julien Grall [this message]
2020-12-09 23:18   ` Stefano Stabellini
2020-12-09 23:35     ` Stefano Stabellini
2020-12-09 23:47       ` Julien Grall
2020-12-10  2:30         ` Stefano Stabellini
2020-12-10 13:17           ` Julien Grall
2020-12-10 13:21           ` Oleksandr
2020-12-09 23:38     ` Julien Grall
2020-11-30 10:31 ` [PATCH V3 16/23] xen/mm: Handle properly reference in set_foreign_p2m_entry() on Arm Oleksandr Tyshchenko
2020-12-08 14:24   ` Jan Beulich
2020-12-08 16:41     ` Oleksandr
2020-12-09 23:49   ` Stefano Stabellini
2021-01-15  1:18   ` Stefano Stabellini
2020-11-30 10:31 ` [PATCH V3 17/23] xen/ioreq: Introduce domain_has_ioreq_server() Oleksandr Tyshchenko
2020-12-08 15:11   ` Jan Beulich
2020-12-08 15:33     ` Oleksandr
2020-12-08 16:56       ` Oleksandr
2020-12-08 19:43         ` Paul Durrant
2020-12-08 20:16           ` Oleksandr
2020-12-09  9:01             ` Paul Durrant
2020-12-09 18:58               ` Julien Grall
2020-12-09 21:05                 ` Oleksandr
2020-12-09 20:36               ` Oleksandr
2020-12-10  8:38                 ` Paul Durrant
2020-12-10 16:57                   ` Oleksandr
2020-11-30 10:31 ` [PATCH V3 18/23] xen/dm: Introduce xendevicemodel_set_irq_level DM op Oleksandr Tyshchenko
2020-12-10  2:21   ` Stefano Stabellini
2020-12-10 12:58     ` Oleksandr
2020-12-10 13:38     ` Julien Grall
2020-11-30 10:31 ` [PATCH V3 19/23] xen/arm: io: Abstract sign-extension Oleksandr Tyshchenko
2020-11-30 21:03   ` Volodymyr Babchuk
2020-11-30 23:27     ` Oleksandr
2020-12-01  7:55       ` Jan Beulich
2020-12-01 10:30         ` Julien Grall
2020-12-01 10:42           ` Oleksandr
2020-12-01 12:13             ` Julien Grall
2020-12-01 12:24               ` Oleksandr
2020-12-01 12:28                 ` Julien Grall
2020-12-01 10:49           ` Jan Beulich
2020-12-01 10:23       ` Julien Grall
2020-11-30 10:31 ` [PATCH V3 20/23] xen/ioreq: Make x86's send_invalidate_req() common Oleksandr Tyshchenko
2020-12-08 15:24   ` Jan Beulich
2020-12-08 16:49     ` Oleksandr
2020-12-09  8:21       ` Jan Beulich
2020-11-30 10:31 ` [PATCH V3 21/23] xen/arm: Add mapcache invalidation handling Oleksandr Tyshchenko
2020-12-10  2:30   ` Stefano Stabellini
2020-12-10 18:50     ` Julien Grall
2020-12-11  1:28       ` Stefano Stabellini
2020-12-11 11:21         ` Oleksandr
2020-12-11 19:07           ` Stefano Stabellini
2020-12-11 19:37             ` Julien Grall
2020-12-11 19:27         ` Julien Grall
2020-11-30 10:31 ` [PATCH V3 22/23] libxl: Introduce basic virtio-mmio support on Arm Oleksandr Tyshchenko
2020-11-30 10:31 ` [PATCH V3 23/23] [RFC] libxl: Add support for virtio-disk configuration Oleksandr Tyshchenko
2020-11-30 11:22 ` [PATCH V3 00/23] IOREQ feature (+ virtio-mmio) on Arm Oleksandr
2020-12-07 13:03   ` Wei Chen
2020-12-07 21:03     ` Oleksandr
2020-11-30 16:21 ` Alex Bennée
2020-11-30 22:22   ` [PATCH V3 00/23] IOREQ feature (+ virtio-mmio) on Arm Oleksandr
2020-12-29 15:32   ` Roger Pau Monné

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