linux-kernel - Re: [PATCH v1 4/4] s390/kvm: VSIE: correctly handle MVPG when in VSIE

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Message-ID: <20210104173644.2e6c8df4@ibm-vm>
Date:   Mon, 4 Jan 2021 17:36:44 +0100
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1 4/4] s390/kvm: VSIE: correctly handle MVPG when in
 VSIE

On Mon, 4 Jan 2021 17:08:15 +0100
David Hildenbrand <david@...hat.com> wrote:

> On 04.01.21 16:22, Claudio Imbrenda wrote:
> > On Sun, 20 Dec 2020 11:13:57 +0100
> > David Hildenbrand <david@...hat.com> wrote:
> >   
> >> On 18.12.20 15:18, Claudio Imbrenda wrote:  
> >>> Correctly handle the MVPG instruction when issued by a VSIE guest.
> >>>     
> >>
> >> I remember that MVPG SIE documentation was completely crazy and
> >> full of corner cases. :)  
> > 
> > you remember correctly
> >   
> >> Looking at arch/s390/kvm/intercept.c:handle_mvpg_pei(), I can spot
> >> that
> >>
> >> 1. "This interception can only happen for guests with DAT disabled
> >> ..." 2. KVM does not make use of any mvpg state inside the SCB.
> >>
> >> Can this be observed with Linux guests?  
> > 
> > a Linux guest will typically not run with DAT disabled
> >   
> >> Can I get some information on what information is stored at [0xc0,
> >> 0xd) inside the SCB? I assume it's:
> >>
> >> 0xc0: guest physical address of source PTE
> >> 0xc8: guest physical address of target PTE  
> > 
> > yes (plus 3 flags in the lower bits of each)  
> 
> Thanks! Do the flags tell us what the deal with the PTE was? If yes,
> what's the meaning of the separate flags?
> 
> I assume something like "invalid, proteced, ??"

bit 61 indicates that the address is a region or segment table entry,
when EDAT applies
bit 62 is "protected" when the protected bit is set in the segment
table entry (or region, if EDAT applies) 
bit 63 is set when the operand was translated with a real-space ASCE

but you can check if the PTE is valid just by dereferencing the
pointers...

> I'm asking because I think we can handle this a little easier.

what is your idea?

> >   
> >> [...]  
> >>>  /*
> >>>   * Run the vsie on a shadow scb and a shadow gmap, without any
> >>> further
> >>>   * sanity checks, handling SIE faults.
> >>> @@ -1063,6 +1132,10 @@ static int do_vsie_run(struct kvm_vcpu
> >>> *vcpu, struct vsie_page *vsie_page) if ((scb_s->ipa & 0xf000) !=
> >>> 0xf000) scb_s->ipa += 0x1000;
> >>>  		break;
> >>> +	case ICPT_PARTEXEC:
> >>> +		if (scb_s->ipa == 0xb254)    
> >>
> >> Old code hat "/* MVPG only */" - why is this condition now
> >> necessary?  
> > 
> > old code was wrong ;)  
> 
> 
> arch/s390/kvm/intercept.c:handle_partial_execution() we only seem to
> handle
> 
> 1. MVPG
> 2. SIGP PEI
> 
> The latter is only relevant for external calls. IIRC, this is only
> active with sigp interpretation - which is never active under vsie
> (ECA_SIGPI).

I think putting an explicit check is better than just a jump in the
dark.