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Re: [Qemu-devel] Cascadelake-Server missing MSR based features ?


From: Tao Xu
Subject: Re: [Qemu-devel] Cascadelake-Server missing MSR based features ?
Date: Tue, 19 Mar 2019 22:35:33 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.5.3

On 3/19/2019 8:16 PM, Daniel P. Berrangé wrote:
The Cascadelake-Server CPU was added last year in

   commit c7a88b52f62b30c04158eeb07f73e3f72221b6a8
   Author: Tao Xu <address@hidden>
   Date:   Wed Sep 19 11:11:22 2018 +0800

     i386: Add new model of Cascadelake-Server

The commit message says that MSR based features are initially
omitted from its definition:

   [quote]
     On Cascadelake, some capabilities (RDCL_NO, IBRS_ALL, RSBA,
     SKIP_L1DFL_VMENTRY and SSB_NO) are enumerated by MSR.
     These features rely on MSR based feature support patch.
     Will be added later after that patch's in.
     http://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg00074.html
   [/quote]

The commits referenced in that URL, however, were merged *before*
this Cascadelake-Server CPU was merged in QEMU:

   commit d86f963694df27f11b3681ffd225c9362de1b634
   Author: Robert Hoo <address@hidden>
   Date:   Mon Oct 15 12:47:25 2018 +0800

     x86: define a new MSR based feature word -- FEATURE_WORDS_ARCH_CAPABILITIES

   commit 07585923485952bf4cb7da563c9f91fecc85d09c
   Author: Robert Hoo <address@hidden>
   Date:   Mon Oct 15 12:47:24 2018 +0800

     x86: Data structure changes to support MSR based features
commit f57bceb6ab5163ddd6c41ff4344ab8cf28a9c63d
   Author: Robert Hoo <address@hidden>
   Date:   Mon Oct 15 12:47:23 2018 +0800

     kvm: Add support to KVM_GET_MSR_FEATURE_INDEX_LIST and KVM_GET_MSRS system 
ioctl
None the less, the Cascadelake-Server CPU in QEMU still does not appear
to have the  RDCL_NO, IBRS_ALL, RSBA, SKIP_L1DFL_VMENTRY and SSB_NO
features present (unless I'm mis-reading the codE).

I see a later commit updated the stepping supposedly to enable
the MSR based features to be exposed but still doesn't appear to have
actually exposed them:

   commit b0a1980384fc265d91de7e09aa5fe531a69e6288
   Author: Tao Xu <address@hidden>
   Date:   Thu Dec 27 10:43:03 2018 +0800

     i386: Update stepping of Cascadelake-Server
Update the stepping from 5 to 6, in order that
     the Cascadelake-Server CPU model can support AVX512VNNI
     and MSR based features exposed by ARCH_CAPABILITIES.

Can anyone shed light on what the status is wrt exposing the MSR based
features in Cascadelake-Server ?

Hi, I sent the patch to add the MSR based features with the Update stepping patch together last year, and we discussed a lot about it:

[1] https://patchwork.kernel.org/patch/10743427/

Tao



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