Re: [Xen-devel] [PATCH v2 20/25] arm/altp2m: Add altp2m paging mechanism.

[Julien Grall <julien.grall@xxxxxxx>]

On 06/08/2016 13:57, Sergej Proskurin wrote:
> Hi Julien,
Hello Sergej,


> On 08/04/2016 06:59 PM, Julien Grall wrote:
> > Hi Sergej,
> >
> > On 01/08/16 18:10, Sergej Proskurin wrote:
> > > diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> > > index 12be7c9..628abd7 100644
> > > --- a/xen/arch/arm/traps.c
> > > +++ b/xen/arch/arm/traps.c
> > [...]
> >
> > > @@ -2403,35 +2405,64 @@ static void do_trap_instr_abort_guest(struct
> > > cpu_user_regs *regs,
> > [...]
> >
> > >      switch ( fsc )
> > >      {
> > > +    case FSC_FLT_TRANS:
> > > +    {
> > > +        if ( altp2m_active(d) )
> > > +        {
> > > +            const struct npfec npfec = {
> > > +                .insn_fetch = 1,
> > > +                .gla_valid = 1,
> > > +                .kind = hsr.iabt.s1ptw ? npfec_kind_in_gpt :
> > > npfec_kind_with_gla
> > > +            };
> > > +
> > > +            /*
> > > +             * Copy the entire page of the failing instruction into the
> > > +             * currently active altp2m view.
> > > +             */
> > > +            if ( altp2m_lazy_copy(v, gpa, gva, npfec, &p2m) )
> > > +                return;
> > I forgot to mention that I think there is a race condition here. If
> > multiple vCPU (let say A and B) use the same altp2m, they may fault here.
> >
> > If vCPU A already fixed the fault, this function will return false and
> > continue. So this will lead to inject an instruction abort to the guest.
> I believe this is exactly what I have experienced in the last days. I
> have applied Tamas' patch [0] but it did not entirely solve the issue. I
> will provide more information about the exact behavior a bit later.
>
> > > +
> > > +            rc = p2m_mem_access_check(gpa, gva, npfec);
> > > +
> > > +            /* Trap was triggered by mem_access, work here is done */
> > > +            if ( !rc )
> > > +                return;
> > > +        }
> > > +
> > > +        break;
> > > +    }
> > [...]
> >
> > > @@ -2470,23 +2503,31 @@ static void do_trap_data_abort_guest(struct
> > > cpu_user_regs *regs,
> > >
> > >      switch ( fsc )
> > >      {
> > > -    case FSC_FLT_PERM:
> > > +    case FSC_FLT_TRANS:
> > >      {
> > > -        const struct npfec npfec = {
> > > -            .read_access = !dabt.write,
> > > -            .write_access = dabt.write,
> > > -            .gla_valid = 1,
> > > -            .kind = dabt.s1ptw ? npfec_kind_in_gpt :
> > > npfec_kind_with_gla
> > > -        };
> > > +        if ( altp2m_active(current->domain) )
> > > +        {
> > > +            const struct npfec npfec = {
> > > +                .read_access = !dabt.write,
> > > +                .write_access = dabt.write,
> > > +                .gla_valid = 1,
> > > +                .kind = dabt.s1ptw ? npfec_kind_in_gpt :
> > > npfec_kind_with_gla
> > > +            };
> > >
> > > -        rc = p2m_mem_access_check(info.gpa, info.gva, npfec);
> > > +            /*
> > > +             * Copy the entire page of the failing data access into the
> > > +             * currently active altp2m view.
> > > +             */
> > > +            if ( altp2m_lazy_copy(v, info.gpa, info.gva, npfec, &p2m) )
> > > +                return;
> > Ditto.
> Ok.
>
> > > +
> > > +            rc = p2m_mem_access_check(info.gpa, info.gva, npfec);
> > > +
> > > +            /* Trap was triggered by mem_access, work here is done */
> > > +            if ( !rc )
> > > +                return;
> > > +        }
> Best regards,
> ~Sergej
>
> [0] https://github.com/tklengyel/xen branch arm_mem_access_reinject
--
Julien Grall

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