Re: [RFC 06/55] KVM: arm64: Add EL2 execution context for nesting

[Christoffer Dall]

On Mon, Jul 03, 2017 at 10:44:51AM -0400, Jintack Lim wrote:
> Thanks Christoffer and Marc,
> 
> On Mon, Jul 3, 2017 at 5:54 AM, Christoffer Dall <cdall@xxxxxxxxxx> wrote:
> > On Mon, Jul 03, 2017 at 10:32:45AM +0100, Marc Zyngier wrote:
> >> On 03/07/17 10:03, Christoffer Dall wrote:
> >> > On Mon, Jun 26, 2017 at 10:33:23AM -0400, Jintack Lim wrote:
> >> >> Hi Christoffer,
> >> >>
> >> >> On Wed, Feb 22, 2017 at 6:10 AM, Christoffer Dall <cdall@xxxxxxxxxx> wrote:
> >> >>> On Mon, Jan 09, 2017 at 01:24:02AM -0500, Jintack Lim wrote:
> >> >>>> With the nested virtualization support, the context of the guest
> >> >>>> includes EL2 register states. The host manages a set of virtual EL2
> >> >>>> registers.  In addition to that, the guest hypervisor supposed to run in
> >> >>>> EL2 is now deprivilaged and runs in EL1. So, the host also manages a set
> >> >>>> of shadow system registers to be able to run the guest hypervisor in
> >> >>>> EL1.
> >> >>>>
> >> >>>> Signed-off-by: Jintack Lim <jintack@xxxxxxxxxxxxxxx>
> >> >>>> Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> >> >>>> ---
> >> >>>>  arch/arm64/include/asm/kvm_host.h | 54 +++++++++++++++++++++++++++++++++++++++
> >> >>>>  1 file changed, 54 insertions(+)
> >> >>>>
> >> >>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >> >>>> index c0c8b02..ed78d73 100644
> >> >>>> --- a/arch/arm64/include/asm/kvm_host.h
> >> >>>> +++ b/arch/arm64/include/asm/kvm_host.h
> >> >>>> @@ -146,6 +146,42 @@ enum vcpu_sysreg {
> >> >>>>       NR_SYS_REGS     /* Nothing after this line! */
> >> >>>>  };
> >> >>>>
> >> >>>> +enum el2_regs {
> >> >>>> +     ELR_EL2,
> >> >>>> +     SPSR_EL2,
> >> >>>> +     SP_EL2,
> >> >>>> +     AMAIR_EL2,
> >> >>>> +     MAIR_EL2,
> >> >>>> +     TCR_EL2,
> >> >>>> +     TTBR0_EL2,
> >> >>>> +     VTCR_EL2,
> >> >>>> +     VTTBR_EL2,
> >> >>>> +     VMPIDR_EL2,
> >> >>>> +     VPIDR_EL2,      /* 10 */
> >> >>>> +     MDCR_EL2,
> >> >>>> +     CNTHCTL_EL2,
> >> >>>> +     CNTHP_CTL_EL2,
> >> >>>> +     CNTHP_CVAL_EL2,
> >> >>>> +     CNTHP_TVAL_EL2,
> >> >>>> +     CNTVOFF_EL2,
> >> >>>> +     ACTLR_EL2,
> >> >>>> +     AFSR0_EL2,
> >> >>>> +     AFSR1_EL2,
> >> >>>> +     CPTR_EL2,       /* 20 */
> >> >>>> +     ESR_EL2,
> >> >>>> +     FAR_EL2,
> >> >>>> +     HACR_EL2,
> >> >>>> +     HCR_EL2,
> >> >>>> +     HPFAR_EL2,
> >> >>>> +     HSTR_EL2,
> >> >>>> +     RMR_EL2,
> >> >>>> +     RVBAR_EL2,
> >> >>>> +     SCTLR_EL2,
> >> >>>> +     TPIDR_EL2,      /* 30 */
> >> >>>> +     VBAR_EL2,
> >> >>>> +     NR_EL2_REGS     /* Nothing after this line! */
> >> >>>> +};
> >> >>>
> >> >>> Why do we have a separate enum and array for the EL2 regs and not simply
> >> >>> expand vcpu_sysreg ?
> >> >>
> >> >> We can expand vcpu_sysreg for the EL2 system registers. For SP_EL2,
> >> >> SPSR_EL2, and ELR_EL2, where is the good place to locate them?.
> >> >> SP_EL1, SPSR_EL1, and ELR_EL1 registers are saved in the kvm_regs
> >> >> structure instead of sysregs[], so I wonder it's better to put them in
> >> >> kvm_regs, too.
> >> >>
> >> >> BTW, what's the reason that those EL1 registers are in kvm_regs
> >> >> instead of sysregs[] in the first place?
> >> >>
> >> >
> >> > This has mostly to do with the way we export things to userspace, and
> >> > for historical reasons.
> >> >
> >> > So we should either expand kvm_regs with the non-sysregs EL2 registers
> >> > and expand sys_regs with the EL2 sysregs, or we should put everything
> >> > EL2 into an EL2 array.  I feel like the first solution will fit more
> >> > nicely into the current design, but I don't have a very strong
> >> > preference.
> >> >
> >> > You should look at the KVM_{GET,SET}_ONE_REG API definition and think
> >> > about how your choice will fit with this.
> >> >
> >> > Marc, any preference?
> >>
> >> My worry is that by changing kvm_regs, we're touching a userspace
> >> visible structure. I'm not sure we can avoid it, but I'd like to avoid
> >> putting too much there (SPSR_EL2 and ELR_EL2 should be enough). I just
> >> had a panic moment when realizing that this structure is not versioned,
> >> but the whole ONE_REG API seems to save us from a complete disaster.
> >>
> >> Overall, having kvm_regs as a UAPI visible thing retrospectively strikes
> >> me as a dangerous design, as we cannot easily expand it. Maybe we should
> >> consider having a kvm_regs_v2 that embeds kvm_regs, and not directly
> >> expose it to userspace (but instead expose the indexes in that
> >> structure)? Userspace that knows how to deal with EL2 will use the new
> >> indexes, while existing SW will carry on using the EL1/EL0 version.
> >
> > We definitely cannot expand kvm_regs, that would lead to all sorts of
> > potential errors, as you correctly point out.
> 
> Ok. I didn't know that kvm_regs are exposed to the user space.
> 
> >
> > So we probably need something like that, or simply let it stay the way
> > it is for now, and add el2_core_regs as a separate thing to the vcpu and
> > only expose the indexes and encoding for those registers?
> >
> 
> Sounds good to me.
> 
> So, expand sys_regs with the EL2 sysregs and put the special purpose
> registers, which is the term used in ARM ARM, such as SPSR_EL2,
> ELR_EL2 and SP_EL2 into el2_core_regs or el2_special_regs, right?
> 

el2_special_regs, yes.

Thanks,
-Christoffer