Re: [RFC PATCH 0/7] x86/entry: Atomic statck switching for IST

[Dave Hansen <dave.hansen@intel.com>]

On 4/3/23 07:05, Lai Jiangshan wrote:
>  Documentation/x86/kernel-stacks.rst   |   2 +
>  arch/x86/entry/Makefile               |   3 +
>  arch/x86/entry/entry_64.S             | 615 +++++++-------------------
>  arch/x86/entry/ist_entry.c            | 299 +++++++++++++
>  arch/x86/include/asm/cpu_entry_area.h |   8 +-
>  arch/x86/include/asm/idtentry.h       |  15 +-
>  arch/x86/include/asm/sev.h            |  14 -
>  arch/x86/include/asm/traps.h          |   1 -
>  arch/x86/kernel/asm-offsets_64.c      |   7 +
>  arch/x86/kernel/callthunks.c          |   2 +
>  arch/x86/kernel/dumpstack_64.c        |   6 +-
>  arch/x86/kernel/nmi.c                 |   8 -
>  arch/x86/kernel/sev.c                 | 108 -----
>  arch/x86/kernel/traps.c               |  43 --
>  arch/x86/kvm/vmx/vmx.c                | 441 +++++++++++++++++-
>  arch/x86/kvm/x86.c                    |  10 +-
>  arch/x86/mm/cpu_entry_area.c          |   2 +-
>  tools/objtool/check.c                 |   7 +-
>  18 files changed, 937 insertions(+), 654 deletions(-)
>  create mode 100644 arch/x86/entry/ist_entry.c

Some high-level comments...

The diffstat looks a lot nastier because of the testing patch.  If you
that patch and trim the diffstat a bit, it starts to look a _lot_ more
appealing:

>  arch/x86/entry/entry_64.S             |  615 ++++++++----------------------------
>  arch/x86/entry/ist_entry.c            |  299 +++++++++++++++++
>  arch/x86/kernel/sev.c                 |  108 ------
>  arch/x86/kernel/traps.c               |   43 --
...
>  16 files changed, 490 insertions(+), 650 deletions(-)

It removes more code than it adds and also removes a bunch of assembly.
If it were me posting this, I'd have shouted that from the rooftops
instead of obscuring it with a testing patch and leaving it as an
exercise to the reader to figure out.

It also comes with testing code and a great cover letter, which are rare
and _spectacular_.

That said, I'm torn.  This series makes a valiant attempt to improve one
of the creakiest corners of arch/x86/.  But, there are precious few
reviewers that can _really_ dig into this stuff.  This is also a lot
less valuable now than it would have been when FRED was not on the horizon.

It's also not incremental at all.  It's going to be a big pain when
someone bisects their random system hang to patch 4/7.  It'll mean
there's a bug buried somewhere in the 500 lines of patch 3/7.

Grumbling aside, there's too much potential upside here to ignore.  As
this moves out of RFC territory, it would be great if you could:

 * Look at the FRED series and determine how much this collides
 * Dig up some reviewers
 * Flesh out the testing story.  What kind of hardware was this tested
   on?  How much was bare metal vs. VMs, etc...?
 * Reinforce what the benefits to end users are here.  Are folks
   _actually_ running into the #VC fragility, for instance?

Also, let's say we queue this, it starts getting linux-next testing, and
we start getting bug reports of hangs.  It'll have to get reverted if we
can't find the bug fast.

How much of a pain would it be to make atomic-IST-entry _temporarily_
selectable at boot time?  It would obviously need to keep the old code
around and would not have the nice diffstat.  But that way, folks would
at least have a workaround while we're bug hunting.

1. https://lore.kernel.org/all/20230327075838.5403-1-xin3.li@intel.com/