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From: Jan Beulich <jbeulich@suse.com>
To: Bertrand Marquis <Bertrand.Marquis@arm.com>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Julien Grall" <julien@xen.org>, "Wei Liu" <wl@xen.org>,
	"paul@xen.org" <paul@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Xia, Hongyan" <hongyxia@amazon.com>,
	xen-devel <xen-devel@lists.xenproject.org>, nd <nd@arm.com>,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [RFC PATCH 1/1] xen: Use a global mapping for runstate
Date: Fri, 29 May 2020 10:45:09 +0200	[thread overview]
Message-ID: <52e26c9d-b662-2597-b521-dacf4f8acfc8@suse.com> (raw)
In-Reply-To: <3B88C76B-6972-4A66-AFDC-0B5C27FBA740@arm.com>

On 29.05.2020 10:13, Bertrand Marquis wrote:
>> On 28 May 2020, at 19:54, Julien Grall <julien@xen.org> wrote:
>> AFAICT, there is no restriction on when the runstate hypercall can be called. So this can even be called before the vCPU is brought up.
> 
> I understand the remark but it still feels very weird to allow an invalid address in an hypercall.
> Wouldn’t we have a lot of potential issues accepting an address that we cannot check ?

I don't think so: The hypervisor uses copy_to_guest() to protect
itself from the addresses to be invalid at the time of copying.
If the guest doesn't make sure they're valid at that time, it
simply won't get the information (perhaps until Xen's next
attempt to copy it out).

You may want to take a look at the x86 side of this (also the
vCPU time updating): Due to the way x86-64 PV guests work, the
address may legitimately be unmapped at the time Xen wants to
copy it, when the vCPU is currently executing guest user mode
code. In such a case the copy gets retried the next time the
guest transitions from user to kernel mode (which involves a
page table change).

Jan


  reply	other threads:[~2020-05-29  8:45 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-28 15:25 [RFC PATCH 0/1] Runstate error with KPTI Bertrand Marquis
2020-05-28 15:25 ` [RFC PATCH 1/1] xen: Use a global mapping for runstate Bertrand Marquis
2020-05-28 16:53   ` Roger Pau Monné
2020-05-28 17:19     ` Bertrand Marquis
2020-05-28 19:12       ` Julien Grall
2020-05-29  8:15         ` Bertrand Marquis
2020-05-28 18:54   ` Julien Grall
2020-05-29  7:19     ` Roger Pau Monné
2020-05-29  8:24       ` Bertrand Marquis
2020-05-29  7:35     ` Jan Beulich
2020-05-29  8:32       ` Bertrand Marquis
2020-05-29  8:37         ` Jan Beulich
2020-05-29 13:26         ` Roger Pau Monné
2020-05-29 13:37           ` Julien Grall
2020-05-29 14:36             ` Roger Pau Monné
2020-05-29 10:59       ` Julien Grall
2020-05-29 13:09         ` Roger Pau Monné
2020-05-29  8:13     ` Bertrand Marquis
2020-05-29  8:45       ` Jan Beulich [this message]
2020-05-29  9:18         ` Bertrand Marquis
2020-05-29  9:27           ` Roger Pau Monné
2020-05-29 13:53             ` Bertrand Marquis
2020-05-29  9:31           ` Jan Beulich
2020-05-29 10:52           ` Julien Grall
2020-05-29  9:43       ` Julien Grall
2020-05-29 14:02         ` Bertrand Marquis
2020-05-29 14:15           ` Julien Grall
2020-05-29 14:21             ` Bertrand Marquis
2020-05-29  9:49       ` Hongyan Xia

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