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

[Julien Grall <julien.grall@xxxxxxx>]

On 28/07/2016 21:48, Tamas K Lengyel wrote:
> On Thu, Jul 28, 2016 at 2:41 PM, Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
> > 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.
> > That isn't my point.  Are there any reserved bits in the registers
> > themselves which could cause Xen to fault when it tries to reload?  If
> > all that happens is a domain_crash() then ok, but if Xen falls over with
> > a fatal fault, that should be avoided.
The TTBR*_EL1 are registers that can be set by the guest without any 
trap to the hypervisor. So they will not cause Xen to fault even writing 
to any reserved bit.
> I agree. At the moment the only register I actually need access
> through vm_event setting is PC so I'll just leave the other registers
> out and document it in the vm_event header.
I am starting to be really annoyed with this kind of sentence. It is not 
difficult to get things correct from the beginning.
You either set/get them or do not expose them at all. But please avoid 
to have half of an implementation just because your use case does not 
need it.
Regards,

--
Julien Grall

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