Re: [PATCH v3] xen/arm: derive GIC CPU interface ID fields from the vGIC

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

Hi Michal,

Thank you for the review.

On Tue, Aug 18, 2026 at 5:48 PM Orzel, Michal <michal.orzel@xxxxxxx> wrote:
>
>
>
> On 18-Aug-26 11:19, Mykola Kvach wrote:
> > Xen exposes ID_AA64PFR0_EL1.GIC and ID_PFR1.GIC from domain_cpuinfo,
> > which is initialized from the sanitized host CPU feature state. This
> > does not necessarily match the virtual interrupt controller configured
> > for a domain.
> >
> > A vGICv2 domain can therefore observe a nonzero GIC field when the host
> > supports the GIC system register interface, even though Xen disables that
> > interface for the domain. On a GICv4.1-capable host, a vGICv3 domain can
> > observe encoding 0b0011, although Xen exposes only its vGICv3 model.
> >
> > Derive the fields from the domain's vGIC version instead. Expose 0b0000
> > for vGICv2 and 0b0001 for vGICv3. This covers ID_AA64PFR0_EL1 and the
> > ID_PFR1_EL1 alias in AArch64 state, as well as ID_PFR1 accessed through
> > CP15 in AArch32 state. Leave the alias unchanged when AArch32 is
> > unavailable.
> >
> > Fixes: 07b9acea116e ("xen/arm: Add handler for ID registers on arm64")
> > Fixes: 8f81064a07c6 ("xen/arm: Add handler for cp15 ID registers")
> > Signed-off-by: Mykola Kvach <mykola_kvach@xxxxxxxx>
> > ---
> > Changes in v3:
> > - Add direct dependencies for the shared ID register helpers.
> > - Move ID_PFR1_GIC_SHIFT to the common CP15 register header.
> > - Apply cosmetic fixes from review.
> >
> > Changes in v2:
> > - Share the GIC ID field helpers between the AArch64 and AArch32 paths.
> > - Parenthesize the individual ASSERT conditions.
> > - Preserve ID_PFR1_EL1.GIC when AArch32 is unavailable.
> > - Target master instead of the 4.22 release.
> >
> > v1: 
> > https://patchew.org/Xen/ba4f779d68c54efc80c4a566dca38ac2e6f9a073.1783675708.git.mykola._5Fkvach@xxxxxxxx/
> > ---
> >  xen/arch/arm/arm64/cpufeature.c          |  1 +
> >  xen/arch/arm/arm64/vsysreg.c             | 18 +++++++++++++++-
> >  xen/arch/arm/include/asm/arm64/sysregs.h |  1 -
> >  xen/arch/arm/include/asm/cpregs.h        |  1 +
> >  xen/arch/arm/include/asm/vreg.h          | 26 ++++++++++++++++++++++++
> >  xen/arch/arm/vcpreg.c                    | 12 ++++++++++-
> >  6 files changed, 56 insertions(+), 3 deletions(-)
> >
> > diff --git a/xen/arch/arm/arm64/cpufeature.c 
> > b/xen/arch/arm/arm64/cpufeature.c
> > index 6fb8974ade..8bae8915e8 100644
> > --- a/xen/arch/arm/arm64/cpufeature.c
> > +++ b/xen/arch/arm/arm64/cpufeature.c
> > @@ -72,6 +72,7 @@
> >  #include <xen/bug.h>
> >  #include <xen/types.h>
> >  #include <xen/kernel.h>
> > +#include <asm/cpregs.h>
> >  #include <asm/sysregs.h>
> >  #include <asm/cpufeature.h>
> >  #include <asm/arm64/cpufeature.h>
> > diff --git a/xen/arch/arm/arm64/vsysreg.c b/xen/arch/arm/arm64/vsysreg.c
> > index d9e3619dfb..4fb50b6972 100644
> > --- a/xen/arch/arm/arm64/vsysreg.c
> > +++ b/xen/arch/arm/arm64/vsysreg.c
> > @@ -20,6 +20,7 @@
> >
> >  #include <asm/arm64/cpufeature.h>
> >  #include <asm/arm64/sve.h>
> > +#include <asm/cpregs.h>
> >  #include <asm/current.h>
> >  #include <asm/regs.h>
> >  #include <asm/traps.h>
> > @@ -298,7 +299,18 @@ void do_sysreg(struct cpu_user_regs *regs,
> >       * to identify the processor features
> >       */
> >      GENERATE_TID3_INFO(ID_PFR0_EL1, pfr32, 0)
> > -    GENERATE_TID3_INFO(ID_PFR1_EL1, pfr32, 1)
> > +    case HSR_SYSREG_ID_PFR1_EL1:
> > +    {
> > +        register_t guest_reg_value = domain_cpuinfo.pfr32.bits[1];
> > +
> > +        if ( cpu_feature64_has_el0_32(&domain_cpuinfo) )
> This check deserves a comment.

Ack.

> > +            guest_reg_value = id_reg_set_gic_field(guest_reg_value,
> > +                                                   ID_PFR1_GIC_SHIFT,
> > +                                                   v->domain);
> > +
> > +        return handle_ro_read_val(regs, regidx, hsr.sysreg.read, hsr, 1,
> > +                                  guest_reg_value);
> > +    }
> >      GENERATE_TID3_INFO(ID_PFR2_EL1, pfr32, 2)
> >      GENERATE_TID3_INFO(ID_DFR0_EL1, dbg32, 0)
> >      GENERATE_TID3_INFO(ID_DFR1_EL1, dbg32, 1)
> > @@ -337,6 +349,10 @@ void do_sysreg(struct cpu_user_regs *regs,
> >              guest_reg_value |= (sysval << ID_AA64PFR0_SVE_SHIFT) & mask;
> >          }
> >
> > +        guest_reg_value = id_reg_set_gic_field(guest_reg_value,
> > +                                               ID_AA64PFR0_GIC_SHIFT,
> > +                                               v->domain);
> > +
> >          return handle_ro_read_val(regs, regidx, hsr.sysreg.read, hsr, 1,
> >                                    guest_reg_value);
> >      }
> > diff --git a/xen/arch/arm/include/asm/arm64/sysregs.h 
> > b/xen/arch/arm/include/asm/arm64/sysregs.h
> > index f3c11d871e..c0a6827b08 100644
> > --- a/xen/arch/arm/include/asm/arm64/sysregs.h
> > +++ b/xen/arch/arm/include/asm/arm64/sysregs.h
> > @@ -438,7 +438,6 @@
> >  #define MVFR1_FPDNAN_SHIFT           4
> >  #define MVFR1_FPFTZ_SHIFT            0
> >
> > -#define ID_PFR1_GIC_SHIFT            28
> >  #define ID_PFR1_VIRT_FRAC_SHIFT      24
> >  #define ID_PFR1_SEC_FRAC_SHIFT       20
> >  #define ID_PFR1_GENTIMER_SHIFT       16
> > diff --git a/xen/arch/arm/include/asm/cpregs.h 
> > b/xen/arch/arm/include/asm/cpregs.h
> > index a7503a190f..79203324fa 100644
> > --- a/xen/arch/arm/include/asm/cpregs.h
> > +++ b/xen/arch/arm/include/asm/cpregs.h
> > @@ -114,6 +114,7 @@
> >  #define MPIDR           p15,0,c0,c0,5   /* Multiprocessor Affinity 
> > Register */
> >  #define ID_PFR0         p15,0,c0,c1,0   /* Processor Feature Register 0 */
> >  #define ID_PFR1         p15,0,c0,c1,1   /* Processor Feature Register 1 */
> > +#define ID_PFR1_GIC_SHIFT 28
> It does not look nice to split CP15 encoding list with this macro. 
> Furthermore,
> it looks a bit odd to move just GIC and not the other fields. Also, how about
> moving the block to asm/sysregs.h? You would not need to include cpregs.h 
> then.

I moved it to cpregs.h following your suggestion in v2 [1].
I will move the whole ID_PFR1_*_SHIFT block to asm/sysregs.h.

>
> >  #define ID_PFR2         p15,0,c0,c3,4   /* Processor Feature Register 2 */
> >  #define ID_DFR0         p15,0,c0,c1,2   /* Debug Feature Register 0 */
> >  #define ID_DFR1         p15,0,c0,c3,5   /* Debug Feature Register 1 */
> > diff --git a/xen/arch/arm/include/asm/vreg.h 
> > b/xen/arch/arm/include/asm/vreg.h
> > index 387ce76e7e..bb6c904f2e 100644
> > --- a/xen/arch/arm/include/asm/vreg.h
> > +++ b/xen/arch/arm/include/asm/vreg.h
> > @@ -4,11 +4,37 @@
> >  #ifndef __ASM_ARM_VREG__
> >  #define __ASM_ARM_VREG__
> >
> > +#include <xen/bitops.h>
> > +#include <xen/bug.h>
> > +#include <xen/sched.h>
> > +
> > +#include <asm/gic.h>
> > +
> >  typedef bool (*vreg_reg64_fn_t)(struct cpu_user_regs *regs, uint64_t *r,
> >                                     bool read);
> >  typedef bool (*vreg_reg_fn_t)(struct cpu_user_regs *regs, register_t *r,
> >                                     bool read);
> >
> > +#define ID_REG_GIC_WIDTH 4
> > +
> > +static inline unsigned int vgic_id_gic_field(const struct domain *d)
> > +{
> > +    ASSERT((d->arch.vgic.version == GIC_V2) ||
> > +           (d->arch.vgic.version == GIC_V3));
> > +
> > +    return d->arch.vgic.version == GIC_V3;
> Wouldn't this be a violation of MISRA C R10.3?

Yes. I will return 1U or 0U explicitly:

    return d->arch.vgic.version == GIC_V3 ? 1U : 0U;

Best regards,
Mykola

[1] 
https://patchew.org/Xen/f46f9e6ebb12d8402fa72b1641796f33e24c539f.1786395761.git.mykola._5Fkvach@xxxxxxxx/#103a2e50-0391-4bad-b9e7-d00c36c76a54@xxxxxxx