Re: [PATCH] KVM: s390: vsie: retry SIE when unable to get vsie_page

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Fri, 16 Jan 2026 10:45:22 -0500
Eric Farman <farman@linux.ibm.com> wrote:

> On Thu, 2026-01-15 at 16:17 -0500, Eric Farman wrote:
> > On Wed, 2026-01-14 at 10:50 +0100, Claudio Imbrenda wrote:  
> > > On Mon, 05 Jan 2026 10:46:53 -0500
> > > Eric Farman <farman@linux.ibm.com> wrote:
> > >   
> > > > On Mon, 2026-01-05 at 13:41 +0100, Janosch Frank wrote:  
> > > > > On 12/17/25 04:01, Eric Farman wrote:    
> > > > > > SIE may exit because of pending host work, such as handling an interrupt,
> > > > > > in which case VSIE rewinds the guest PSW such that it is transparently
> > > > > > resumed (see Fixes tag). There is still one scenario where those conditions  
> > > 
> > > can you add a few words to (very briefly) explain what the scenario is?  
> > 
> > Maybe if this paragraph were rewritten this way, instead?
> > 
> > --8<--
> > SIE may exit because of pending host work, such as handling an interrupt,
> > in which case VSIE rewinds the guest PSW such that it is transparently
> > resumed (see Fixes tag). Unlike those other places that return rc=0, this
> > return leaves the guest PSW in place, requiring the guest to handle an
> > intercept that was meant to be serviced by the host. This showed up when
> > testing heavy I/O workloads, when multiple vcpus attempted to dispatch the
> > same SIE block and incurred failures inserting them into the radix tree.  
> > -->8--  
> 
> Spoke to Claudio offline, and he suggested the following edit to the above:
> 
> --8<--
> SIE may exit because of pending host work, such as handling an interrupt,
> in which case VSIE rewinds the guest PSW such that it is transparently
> resumed (see Fixes tag). Unlike those other places that return rc=0, this
> return leaves the guest PSW in place, requiring the guest to handle a
> spurious intercept. This showed up when testing heavy I/O workloads,
> when multiple vcpus attempted to dispatch the same SIE block and incurred
> failures inserting them into the radix tree.
> -->8--  

with the above text:

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>


> 
> > 
> > @Janosch, if that ends up being okay, can you update the patch or do you want me to send a v2?
> >   
> > >   
> > > > > > are not present, but that the VSIE processor returns with effectively rc=0,
> > > > > > resulting in additional (and unnecessary) guest work to be performed.
> > > > > > 
> > > > > > For this case, rewind the guest PSW as we do in the other non-error exits.
> > > > > > 
> > > > > > Fixes: 33a729a1770b ("KVM: s390: vsie: retry SIE instruction on host intercepts")
> > > > > > Signed-off-by: Eric Farman <farman@linux.ibm.com>    
> > > > > 
> > > > > This is purely cosmetic to have all instances look the same, right?    
> > > > 
> > > > Nope, I can take this path with particularly high I/O loads on the system, which ends up
> > > > (incorrectly) sending the intercept to the guest.  
> > > 
> > > this is a good candidate for the explanation I mentioned above :)
> > > 
> > > 
> > > (the patch itself looks fine)