linux-kernel - Re: [Kernel BUG?] SMSW operation get success on UMIP KVM guest

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Message-ID: <20190701160352.GA19921@ranerica-svr.sc.intel.com>
Date:   Mon, 1 Jul 2019 09:03:52 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Li Wang <liwang@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, tglx@...utronix.de,
        kernellwp@...il.com, ricardo.neri@...el.com, pengfei.xu@...el.com,
        LTP List <ltp@...ts.linux.it>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ping Fang <pifang@...hat.com>
Subject: Re: [Kernel BUG?] SMSW operation get success on UMIP KVM guest

On Mon, Jul 01, 2019 at 08:57:28PM +0800, Li Wang wrote:
> On Mon, Jul 1, 2019 at 8:02 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> > On 01/07/19 09:50, Li Wang wrote:
> > > Hello there,
> > >
> > > LTP/umip_basic_test get failed on KVM UMIP
> > > system(kernel-v5.2-rc4.x86_64). The test is only trying to do
> > >      asm volatile("smsw %0\n" : "=m" (val));
> > > and expect to get SIGSEGV in this SMSW operation, but it exits with 0
> > > unexpectedly.
> >
> > In addition to what Thomas said, perhaps you are using a host that does
> > *not* have UMIP, and configuring KVM to emulate it(*).  In that case, it
> > is not possible to intercept SMSW, and therefore it will incorrectly
> > succeed.
> >
> 
> Right, I checked the host system, and confirmed that CPU doesn't support
> UMIP.
> 
> >
> > Paolo
> >
> > (*) before the x86 people jump at me, this won't happen unless you
> > explicitly pass an option to QEMU, such as "-cpu host,+umip". :)  The
> > incorrect emulation of SMSW when CR4.UMIP=1 is why.
> >
> Good to know this, is there any document for that declaration? It seems
> neither LTP issue nor kernel bug here. But anyway we'd better do something
> to avoid the error in the test.

The test case already checks for umip in /proc/cpuinfo, right? And in
long mode it always expects a SIGSEGV signal. If you did not add -cpu host,+umip,
how come umip was present in /proc/cpuinfo?

Thanks and BR,
Ricardo