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From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <Andrew.Cooper3@citrix.com>
Cc: Roger Pau Monne <roger.pau@citrix.com>, Wei Liu <wl@xen.org>,
	Xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v3 7/9] x86/svm: VMEntry/Exit logic for MSR_SPEC_CTRL
Date: Tue, 1 Feb 2022 13:40:11 +0100	[thread overview]
Message-ID: <52587ee4-56a3-3824-75cd-667c35e3109f@suse.com> (raw)
In-Reply-To: <e48536c0-14c1-1d41-0a9e-4fc0f0aa9c36@citrix.com>

On 01.02.2022 13:28, Andrew Cooper wrote:
> On 01/02/2022 11:47, Jan Beulich wrote:
>> On 31.01.2022 16:36, Andrew Cooper wrote:
>>> Hardware maintains both host and guest versions of MSR_SPEC_CTRL, but guests
>>> run with the logical OR of both values.  Therefore, in principle we want to
>>> clear Xen's value before entering the guest.  However, for migration
>>> compatibility,
>> I think you've explained this to me before, but I can't seem to put
>> all of it together already now. Could expand on how a non-zero value
>> behind a guest's back can help with migration compatibility? At the
>> first glance I would be inclined to say only what the guest actually
>> gets to see and use can affect its migration.
> 
> For VMs which see VIRT_SPEC_CTRL (compatibility with Fam15 thru Zen1),
> writes of VIRT_SPEC_CTRL.SSBD (probably) need to use
> SSBD-behind-the-guest's back.  I say probably, because I think this is
> the least bad implementation option, but until we have working support,
> it's still a guess.

So this is future work (and mentioning just this in the description
would be enough to address my comment), but ...

> For the ultra paranoid, a VM migrating in which can't see PSFD (e.g. for
> compatibility with Zen2) should have PSFD set behind it's back.

... this is something we should be doing right away then?

Jan



  reply	other threads:[~2022-02-01 12:40 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-28 13:29 [PATCH v2 0/9] x86: MSR_SPEC_CTRL support for SVM guests Andrew Cooper
2022-01-28 13:29 ` [PATCH v2 1/9] x86/cpuid: Advertise SSB_NO to guests by default Andrew Cooper
2022-01-28 14:31   ` Jan Beulich
2022-01-31  9:41   ` Roger Pau Monné
2022-01-31 11:15     ` Andrew Cooper
2022-01-31 11:23       ` Roger Pau Monné
2022-01-28 13:29 ` [PATCH v2 2/9] x86/spec-ctrl: Drop use_spec_ctrl boolean Andrew Cooper
2022-01-28 13:29 ` [PATCH v2 3/9] x86/spec-ctrl: Introduce new has_spec_ctrl boolean Andrew Cooper
2022-01-28 13:29 ` [PATCH v2 4/9] x86/spec-ctrl: Don't use spec_ctrl_{enter,exit}_idle() for S3 Andrew Cooper
2022-01-29  1:09   ` Andrew Cooper
2022-01-31 10:15   ` Jan Beulich
2022-01-31 11:23     ` Andrew Cooper
2022-01-31 14:24       ` Andrew Cooper
2022-01-28 13:29 ` [PATCH v2 5/9] x86/spec-ctrl: Record the last write to MSR_SPEC_CTRL Andrew Cooper
2022-01-31 10:20   ` Jan Beulich
2022-01-31 11:35     ` Andrew Cooper
2022-01-28 13:29 ` [PATCH v2 6/9] x86/spec-ctrl: Use common MSR_SPEC_CTRL logic for AMD Andrew Cooper
2022-01-31 10:25   ` Jan Beulich
2022-01-28 13:29 ` [PATCH v2 7/9] x86/svm: VMEntry/Exit logic for MSR_SPEC_CTRL Andrew Cooper
2022-01-31 10:33   ` Jan Beulich
2022-01-31 11:47     ` Andrew Cooper
2022-01-31 12:55   ` Jan Beulich
2022-01-31 14:04     ` Andrew Cooper
2022-01-31 15:36   ` [PATCH v3 " Andrew Cooper
2022-02-01 11:47     ` Jan Beulich
2022-02-01 12:28       ` Andrew Cooper
2022-02-01 12:40         ` Jan Beulich [this message]
2022-02-01 12:46           ` Andrew Cooper
2022-01-28 13:29 ` [PATCH v2 8/9] x86/msr: AMD MSR_SPEC_CTRL infrastructure Andrew Cooper
2022-01-28 13:29 ` [PATCH v2 9/9] x86/cpuid: Enable MSR_SPEC_CTRL in SVM guests by default Andrew Cooper
2022-01-31 10:39   ` Jan Beulich
2022-01-31 11:54     ` Andrew Cooper

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