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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "George Dunlap" <george.dunlap@citrix.com>,
	"Ian Jackson" <iwj@xenproject.org>,
	"Julien Grall" <julien@xen.org>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v3 03/22] x86/xstate: re-size save area when CPUID policy changes
Date: Tue, 11 May 2021 17:41:54 +0100	[thread overview]
Message-ID: <f16afc8a-ccd4-7e5e-e08d-d96597c6e8ab@citrix.com> (raw)
In-Reply-To: <f515fdfb-d1a6-56d8-5db3-ebddeed23806@suse.com>

On 03/05/2021 15:22, Jan Beulich wrote:
>> Another consequence is that we need to rethink our hypercall behaviour. 
>> There is no such thing as supervisor states in an uncompressed XSAVE
>> image, which means we can't continue with that being the ABI.
> I don't think the hypercall input / output blob needs to follow any
> specific hardware layout.

Currently, the blob is { xcr0, xcr0_accum, uncompressed image }.

As we haven't supported any compressed states yet, we are at liberty to
create a forward compatible change by logically s/xcr0/xstate/ and
permitting an uncompressed image.

Irritatingly, we have xcr0=0 as a permitted state and out in the field,
for "no xsave state".  This contributes a substantial quantity of
complexity in our xstate logic, and invalidates the easy fix I had for
not letting the HVM initpath explode.

The first task is to untangle the non-architectural xcr0=0 case, and to
support compressed images.  Size parsing needs to be split into two, as
for compressed images, we need to consume XSTATE_BV and XCOMP_BV to
cross-check the size.

I think we also want a rule that Xen will always send compressed if it
is using XSAVES (/XSAVEC in the interim?)  We do not want to be working
with uncompressed images at all, now that MPX is a reasonable sized hole
in the middle.

Cleaning this up will then unblock v2 of the existing xstate cleanup
series I posted.

>> In terms of actual context switching, we want to be using XSAVES/XRSTORS
>> whenever it is available, even if we're not using supervisor states. 
>> XSAVES has both the inuse and modified optimisations, without the broken
>> consequence of XSAVEOPT (which is firmly in the "don't ever use this"
>> bucket now).
> The XSAVEOPT anomaly is affecting user mode only, isn't it? Or are
> you talking of something I have forgot about?

It's not safe to use at all in L1 xen, because the tracking leaks
between non-root contexts.  I can't remember if there are further
problems for an L0 xen, but I have a nagging feeling that there is.

~Andrew



  reply	other threads:[~2021-05-11 16:42 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-22 14:38 [PATCH v3 00/22] xvmalloc() / x86 xstate area / x86 CPUID / AMX+XFD Jan Beulich
2021-04-22 14:43 ` [PATCH v3 01/22] mm: introduce xvmalloc() et al and use for grant table allocations Jan Beulich
2021-05-03 11:31   ` Roger Pau Monné
2021-05-03 13:50     ` Jan Beulich
2021-05-03 14:54       ` Roger Pau Monné
2021-05-03 15:21         ` Jan Beulich
2021-05-03 16:39           ` Roger Pau Monné
2021-04-22 14:44 ` [PATCH v3 02/22] x86/xstate: use xvzalloc() for save area allocation Jan Beulich
2021-05-05 13:29   ` Roger Pau Monné
2021-04-22 14:44 ` [PATCH v3 03/22] x86/xstate: re-size save area when CPUID policy changes Jan Beulich
2021-05-03 13:57   ` Andrew Cooper
2021-05-03 14:22     ` Jan Beulich
2021-05-11 16:41       ` Andrew Cooper [this message]
2021-05-17  7:33         ` Jan Beulich
2021-04-22 14:45 ` [PATCH v3 04/22] x86/xstate: re-use valid_xcr0() for boot-time checks Jan Beulich
2021-05-03 11:53   ` Andrew Cooper
2021-04-22 14:45 ` [PATCH v3 05/22] x86/xstate: drop xstate_offsets[] and xstate_sizes[] Jan Beulich
2021-05-03 16:10   ` Andrew Cooper
2021-05-04  7:57     ` Jan Beulich
2021-04-22 14:46 ` [PATCH v3 06/22] x86/xstate: replace xsave_cntxt_size and drop XCNTXT_MASK Jan Beulich
2021-04-22 14:47 ` [PATCH v3 07/22] x86/xstate: avoid accounting for unsupported components Jan Beulich
2021-04-22 14:47 ` [PATCH v3 08/22] x86: use xvmalloc() for extended context buffer allocations Jan Beulich
2021-04-22 14:48 ` [PATCH v3 09/22] x86/xstate: enable AMX components Jan Beulich
2021-04-22 14:50 ` [PATCH v3 10/22] x86/CPUID: adjust extended leaves out of range clearing Jan Beulich
2021-04-22 14:50 ` [PATCH v3 11/22] x86/CPUID: move bounding of max_{,sub}leaf fields to library code Jan Beulich
2021-04-22 14:51 ` [PATCH v3 12/22] x86/CPUID: enable AMX leaves Jan Beulich
2021-04-22 14:52 ` [PATCH v3 13/22] x86: XFD enabling Jan Beulich
2021-04-22 14:53 ` [PATCH v3 14/22] x86emul: introduce X86EMUL_FPU_{tilecfg,tile} Jan Beulich
2021-04-22 14:53 ` [PATCH v3 15/22] x86emul: support TILERELEASE Jan Beulich
2021-04-22 14:53 ` [PATCH v3 16/22] x86: introduce struct for TILECFG register Jan Beulich
2021-04-22 14:54 ` [PATCH v3 17/22] x86emul: support {LD,ST}TILECFG Jan Beulich
2021-04-22 14:55 ` [PATCH v3 18/22] x86emul: support TILEZERO Jan Beulich
2021-04-22 14:55 ` [PATCH v3 19/22] x86emul: support TILELOADD{,T1} and TILESTORE Jan Beulich
2021-04-22 15:06   ` Jan Beulich
2021-04-22 15:11     ` Jan Beulich
2021-04-26  7:12       ` Paul Durrant
2021-04-29  9:40         ` Jan Beulich
2021-04-22 14:56 ` [PATCH v3 20/22] x86emul: support tile multiplication insns Jan Beulich
2021-04-22 14:57 ` [PATCH v3 21/22] x86emul: test AMX insns Jan Beulich
2021-04-22 14:57 ` [PATCH v3 22/22] x86: permit guests to use AMX and XFD Jan Beulich

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