Re: [PATCH 1/2] x86/mm: Change default value for suppress #VE in set_mem_access()

From: Tamas K Lengyel <tamas@tklengyel.com>
To: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>,
	Vlad Ioan Topan <itopan@bitdefender.com>,
	Razvan Cojocaru <rcojocaru@bitdefender.com>,
	Adrian Pop <apop@bitdefender.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Jan Beulich <jbeulich@suse.com>,
	Xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH 1/2] x86/mm: Change default value for suppress #VE in set_mem_access()
Date: Thu, 20 Jul 2017 10:14:35 -0600	[thread overview]
Message-ID: <CABfawh=oi0++4D-38tHJzS4w85AMiuagdA9o3y9eDO1abcuHdg@mail.gmail.com> (raw)
In-Reply-To: <CAFLBxZaqUFv1givvNTZ808y8rFPpM+Nk_cJdp8uYjSCaPD4a6g@mail.gmail.com>

On Thu, Jul 20, 2017 at 8:38 AM, George Dunlap
<George.Dunlap@eu.citrix.com> wrote:
> On Thu, Jun 15, 2017 at 7:49 PM, Tamas K Lengyel <tamas@tklengyel.com> wrote:
>> On Fri, Jun 9, 2017 at 10:51 AM, Adrian Pop <apop@bitdefender.com> wrote:
>>> From: Vlad Ioan Topan <itopan@bitdefender.com>
>>>
>>> The default value for the "suppress #VE" bit set by set_mem_access()
>>> currently depends on whether the call is made from the same domain (the
>>> bit is set when called from another domain and cleared if called from
>>> the same domain). This patch changes that behavior to inherit the old
>>> suppress #VE bit value if it is already set and to set it to 1
>>> otherwise, which is safer and more reliable.
>>
>> Could you elaborate on why do you think it is safer and more reliable
>> to switch the behavior? I believe the original idea was that the
>> domain should only be allowed to clear an SVE bit set by an external
>> tool. With this change it will allow the guest to request VE for any
>> page the external tool hasn't itself reserved specifically.
>
> Hmm?  This patch by itself simply prevents the guest from changing the
> VE bit at all (either setting or clearing it).
>
> Or did you mean, "This patch series"?

No, technically the other patch is fine by itself. It can only be used
to set the SVE bit from a privileged domain, but by itself that is
fine. Only this patch is problematic if we want to allow a setup where
there is only an in-guest tool without a corresponding vm_event
mem_access listener.

Tamas

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