Re: [Xen-devel] [PATCH v3] xen: introduce VCPUOP_register_runstate_phys_memory_area hypercall

From: "Jan Beulich" <JBeulich@suse.com>
To: <andrii.anisov@gmail.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <Ian.Jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	Julien Grall <julien.grall@arm.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	"andrii_anisov@epam.com" <andrii_anisov@epam.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v3] xen: introduce VCPUOP_register_runstate_phys_memory_area hypercall
Date: Thu, 13 Jun 2019 06:39:07 -0600	[thread overview]
Message-ID: <5D0243EB0200007800237D74@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <f8f8ea51-46dd-7e0a-9b5b-4da439eee4c0@gmail.com>

>>> On 13.06.19 at 14:21, <andrii.anisov@gmail.com> wrote:
> On 11.06.19 13:22, Andrii Anisov wrote:
>> On 11.06.19 12:10, Jan Beulich wrote:
>>>>> Except when there's no need for KPTI in the guest in the first place,
>>>>> as is the case for x86-64 PV guests. I think this is worthwhile clarifying.
>>>>
>>>> I am not sure what is your point here. At least on Arm, using virtual address is
>>>> not safe at all (whether KPTI is used or not). A guest can genuinely decides to
>>>> shatter the mapping where the virtual address is. On Arm, this require to use
>>>> the break-before-make sequence. It means the translation VA -> PA may fail is
>>>> you happen to do it while the guest is using the sequence.
>>>>
>>>> Some of the intermittent issues I have seen on the Arndale in the past [1] might
>>>> be related to using virtual address. I am not 100% sure because even if the
>>>> debug, the error does not make sense. But this is the most plausible reason for
>>>> the failure.
>>>
>>> All fine, but Arm-specific. The point of my comment was to ask to call
>>> out that there is one environment (x86-64 PV) where this KPTI
>>> discussion is entirely inapplicable.
>> 
>> I admit that x86 specifics are quite unclear to me so clarifications and 
> corrections in that domain are desirable.
> 
> Could you please elaborate more about this?
> Do you mean that more words should be added to the commit message about x86?
> If so, please provide what is proper from your point of view.

I still think my initial response (still visible in context) was
sufficient. All I'm after is that you slightly soften your bold
statement in the description (no longer visible in context).

Jan

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