Re: [PATCH v2] x86/entry: Avoid redundant CR3 write on paranoid returns

[Peter Zijlstra]

On Wed, Sep 20, 2023 at 03:04:43PM +0000, Brendan Jackman wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> 
> This path gets used called from:
> 
> 1. #NMI return.
> 2. paranoid_exit (i.e. #MCE, #VC, #DB and #DF return)
> 
> Contrary to the implication in commit 21e94459110252 ("x86/mm: Optimize
> RESTORE_CR3"), we never modify CR3 in any of these exceptions, except
> for switching from user to kernel pagetables under PTI. That means that
> most of the time when returning from an exception that interrupted the
> kernel no CR3 restore is necessary. Writing CR3 is expensive on some
> machines, so this commit avoids redundant writes.
> 
> I said "most of the time" because we might have interrupted the kernel
> entry before the user->kernel CR3 switch or the exit after the
> kernel->user switch. In the former case skipping the restore might
> actually be be fine, but definitely not the latter. So we do still need
> to check the saved CR3 and restore it if it's a user CR3.
> 
> To reflect the new behaviour RESTORE_CR3 is given a longer name, and a
> comment that was describing its behaviour at the call site is removed.
> We can also simplify the code around the SET_NOFLUSH_BIT invocation
> as we no longer need to branch to it from above.
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> [Rewrote commit message; responded to review comments]
> Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> ---

Seems sensible, although I do wonder what made you care enough to
optimize the PTI paranoid path... :-)

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>