Re: [PATCH 9/9] xen/arm64: Remove READ/WRITE_SYSREG32 helper macros

[Julien Grall <julien@xxxxxxx>]

On 27/04/2021 08:16, Michal Orzel wrote:
> Hi Jilien,
Hi,

> On 21.04.2021 21:16, Julien Grall wrote:
> > Hi Michal,
> >
> > On 20/04/2021 08:08, Michal Orzel wrote:
> > > AArch64 system registers are 64bit whereas AArch32 ones
> > > are 32bit or 64bit. MSR/MRS are expecting 64bit values thus
> > > we should get rid of helpers READ/WRITE_SYSREG32
> > > in favour of using READ/WRITE_SYSREG.
> > > We should also use register_t type when reading sysregs
> > > which can correspond to uint64_t or uint32_t.
> > > Even though many AArch64 sysregs have upper 32bit reserved
> > > it does not mean that they can't be widen in the future.
> > As a general comment, all your commit message contains information about the 
> > goal (which is great). But they don't go much in details about the actual 
> > changes. I have tried to point out where I think those details would be helpful.
> >
> > > As there are no other places in the code using READ/WRITE_SYSREG32,
> > > remove the helper macros.
> > >
> > > Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
> > > ---
> > >    xen/arch/arm/vcpreg.c               | 16 ++++++++++++++++
> > >    xen/include/asm-arm/arm64/sysregs.h |  5 -----
> > >    2 files changed, 16 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/xen/arch/arm/vcpreg.c b/xen/arch/arm/vcpreg.c
> > > index c7f516ee0a..6cb7f3108c 100644
> > > --- a/xen/arch/arm/vcpreg.c
> > > +++ b/xen/arch/arm/vcpreg.c
> > > @@ -48,6 +48,7 @@
> > >     */
> > >      /* The name is passed from the upper macro to workaround macro expansion. 
> > > */
> > > +#ifdef CONFIG_ARM_32
> > >    #define TVM_REG(sz, func, reg...)                                           \
> > >    static bool func(struct cpu_user_regs *regs, uint##sz##_t *r, bool read)    \
> > >    {                                                                           \
> > > @@ -61,6 +62,21 @@ static bool func(struct cpu_user_regs *regs, uint##sz##_t *r, bool 
> > > read)    \
> > >                                                                                \
> > >        return true;                                                            \
> > >    }
> > > +#else /* CONFIG_ARM_64 */
> > > +#define TVM_REG(sz, func, reg...)                                           \
> > > +static bool func(struct cpu_user_regs *regs, uint##sz##_t *r, bool read)    \
> > > +{                                                                           \
> > > +    struct vcpu *v = current;                                               \
> > > +    bool cache_enabled = vcpu_has_cache_enabled(v);                         \
> > > +                                                                            \
> > > +    GUEST_BUG_ON(read);                                                     \
> > > +    WRITE_SYSREG(*r, reg);                                                  \
> > > +                                                                            \
> > > +    p2m_toggle_cache(v, cache_enabled);                                     \
> > > +                                                                            \
> > > +    return true;                                                            \
> > > +}
> > > +#endif
> > It would be nice if this change can be explained in the commit message. 
> > However, I think we can avoid the duplication by aliasing TVM_REG32() to 
> > TVM_REG64() on Arm64.
> Unfortunately we cannot. This is the only working solution for now.
> If we do the alias we will get many errors due to incompatbile pointer type in 
> vreg_emulate_*.
> Without a proper change in this functions we can't do that.
Right, so the prototype needs to stay the same. How about provide a 
wrapper WRITE_SYSREG_SZ(sz, val, sysreg) internal to vcpreg.c?
On 32-bit it would expand to WRITE_SYSREG##sz(val, sysreg) and on 
64-bit, it would expand to WRITE_SYSREG().
> I will modify it once I start working on vreg_emulate topic but it requires 
> more investigation.
While it would be great to get rid of {READ, WRITE}_SYSREG32, I don't 
want to duplicate the code (even temporarily) just for the sake for 
removing the helpers.
If the new proposal I suggested above doesn't work, then I would 
strongly prefer to keep the existing code until vreg_emulate_* is reworked.
Cheers,

--
Julien Grall