Re: [Xen-devel] [PATCH] arm/vm_event: get/set registers

[Julien Grall <julien.grall@xxxxxxx>]

On 28/07/2016 21:36, Tamas K Lengyel wrote:
> On Thu, Jul 28, 2016 at 2:26 PM, Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 28/07/2016 21:05, Tamas K Lengyel wrote:
> > > Add support for getting/setting registers through vm_event on ARM.
> > > The set of registers can be expanded in the future to include other registers
> > > as well if necessary but for now it is limited to TTB/CR/R0/R1, PC and CPSR.
> > >
> > > Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
> > > ---
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Cc: Julien Grall <julien.grall@xxxxxxx>
> > > Cc: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > For the x86 and common bits, Reviewed-by: Andrew Cooper
> > <andrew.cooper3@xxxxxxxxxx>
> >
> > However,
> >
> > > +#include <xen/sched.h>
> > > +#include <asm/vm_event.h>
> > > +
> > > +void vm_event_fill_regs(vm_event_request_t *req)
> > > +{
> > > +    const struct cpu_user_regs *regs = guest_cpu_user_regs();
> > > +
> > > +    req->data.regs.arm.cpsr = regs->cpsr;
> > > +    req->data.regs.arm.pc = regs->pc;
> > > +    req->data.regs.arm.ttbcr = READ_SYSREG(TCR_EL1);
> > > +    req->data.regs.arm.ttbr0 = READ_SYSREG64(TTBR0_EL1);
> > > +    req->data.regs.arm.ttbr1 = READ_SYSREG64(TTBR1_EL1);
> > > +}
> > > +
> > > +void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp)
> > > +{
> > > +    struct cpu_user_regs *regs = guest_cpu_user_regs();
> > > +
> > > +    regs->cpsr = rsp->data.regs.arm.cpsr;
> > > +    regs->pc = rsp->data.regs.arm.pc;
> > > +    v->arch.ttbcr = rsp->data.regs.arm.ttbcr;
> > > +    v->arch.ttbr0 = rsp->data.regs.arm.ttbr0;
> > > +    v->arch.ttbr1 = rsp->data.regs.arm.ttbr1;
> > Not knowing anything about ARM, but this looks like it is missing some
> > sanity/plausibility checks (to protect Xen against accidental clobbering
> > from the vm_event listener), and some WRITE_SYSREG() to reload the new
> > values (unless this is done unconditionally later, at which point you
> > should at least leave a comment here saying so).
> This function only ever gets called if the vm_event response
> specifically has the VM_EVENT_FLAG_SET_REGISTERS set, so accidental
> clobbering is not possible. Also, using WRITE_SYSREG() is not safe at
> this point because current != v. However, I have another issue here
> with regs which should actually be:
What if the vCPU is running? If it cannot happen, please document and 
add an ASSERT/BUG_ON.
>     struct cpu_user_regs *regs = &v->arch.cpu_info->guest_cpu_user_regs;
>
> I'll fix that shortly.
>
> Thanks,
> Tamas
--
Julien Grall

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