Re: [Xen-devel] [PATCH] xen: do live patching only from main idle loop

From: Ross Lagerwall <ross.lagerwall@citrix.com>
To: Jan Beulich <jbeulich@suse.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: "Juergen Gross" <jgross@suse.com>,
	"Kevin Tian" <kevin.tian@intel.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Julien Grall" <julien@xen.org>, "Wei Liu" <wl@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Jun Nakajima" <jun.nakajima@intel.com>,
	xen-devel@lists.xenproject.org,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH] xen: do live patching only from main idle loop
Date: Mon, 2 Mar 2020 14:41:46 +0000	[thread overview]
Message-ID: <680f7622-3e4b-1049-cecc-527910739ca9@citrix.com> (raw)
In-Reply-To: <6b65b34d-0078-ad0c-9a6c-ada5d2828cc0@suse.com>

On 3/2/20 2:28 PM, Jan Beulich wrote:
> On 11.02.2020 10:31, Juergen Gross wrote:
>> One of the main design goals of core scheduling is to avoid actions
>> which are not directly related to the domain currently running on a
>> given cpu or core. Live patching is one of those actions which are
>> allowed taking place on a cpu only when the idle scheduling unit is
>> active on that cpu.
>>
>> Unfortunately live patching tries to force the cpus into the idle loop
>> just by raising the schedule softirq, which will no longer be
>> guaranteed to work with core scheduling active. Additionally there are
>> still some places in the hypervisor calling check_for_livepatch_work()
>> without being in the idle loop.
>>
>> It is easy to force a cpu into the main idle loop by scheduling a
>> tasklet on it. So switch live patching to use tasklets for switching to
>> idle and raising scheduling events. Additionally the calls of
>> check_for_livepatch_work() outside the main idle loop can be dropped.
>>
>> As tasklets are only running on idle vcpus and stop_machine_run()
>> is activating tasklets on all cpus but the one it has been called on
>> to rendezvous, it is mandatory for stop_machine_run() to be called on
>> an idle vcpu, too, as otherwise there is no way for scheduling to
>> activate the idle vcpu for the tasklet on the sibling of the cpu
>> stop_machine_run() has been called on.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>  xen/arch/arm/domain.c       |  9 ++++-----
>>  xen/arch/arm/traps.c        |  6 ------
>>  xen/arch/x86/domain.c       |  9 ++++-----
>>  xen/arch/x86/hvm/svm/svm.c  |  2 +-
>>  xen/arch/x86/hvm/vmx/vmcs.c |  2 +-
>>  xen/arch/x86/pv/domain.c    |  2 +-
>>  xen/arch/x86/setup.c        |  2 +-
>>  xen/common/livepatch.c      | 39 ++++++++++++++++++++++++++++++++++-----
>>  8 files changed, 46 insertions(+), 25 deletions(-)
> 
> Konrad, Ross - I was about to apply this when I noticed an ack
> by one of the two of you is still needed. Care to provide one
> (or comment if there are issues)?
> 

Sorry.

Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>

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