Re: [PATCH 7/8] x86/public: Split the struct cpu_user_regs type

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Mar 24, 2025 at 10:47:29AM +0100, Jan Beulich wrote:
> On 21.03.2025 16:11, Andrew Cooper wrote:
> > On 17/03/2025 12:15 pm, Jan Beulich wrote:
> >> On 11.03.2025 22:10, Andrew Cooper wrote:
> >>> In order to support FRED, we're going to have to remove the {ds..gs} 
> >>> fields
> >>> from struct cpu_user_regs, meaning that it is going to have to become a
> >>> different type to the structure embedded in vcpu_guest_context_u.
> >>>
> >>> struct cpu_user_regs is a name used in common Xen code (i.e. needs to stay
> >>> using this name), so renaming the public struct to be guest_user_regs in 
> >>> Xen's
> >>> view only.
> >>>
> >>> Introduce a brand hew cpu-user-regs.h, currently containing a duplicate
> >>> structure.  This removes the need for current.h to include public/xen.h, 
> >>> and
> >>> highlights a case where the emulator was picking up cpu_user_regs
> >>> transitively.
> >>>
> >>> No functional change.
> >>>
> >>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> >> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > 
> > Thanks.
> > 
> >>> cpu_user_regs_t and the guest handle don't seem to be used anywhere.  I'm
> >>> tempted to exclude them from Xen builds.
> >> I concur. We can always re-expose them should they be needed somewhere.
> > 
> > It's actually a little ugly to do.
> > 
> > #ifdef __XEN__
> > #undef cpu_user_regs
> > #else
> > typedef struct cpu_user_regs cpu_user_regs_t;
> > DEFINE_XEN_GUEST_HANDLE(cpu_user_regs_t);
> > #endif
> > 
> > and I don't particularly like it, given the complexity of #ifdef-ary
> > around it.  Thoughts?
> 
> It's not really pretty, but I'd be okay with this.
> 
> >>> --- /dev/null
> >>> +++ b/xen/arch/x86/include/asm/cpu-user-regs.h
> >>> @@ -0,0 +1,69 @@
> >>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> >>> +#ifndef X86_CPU_USER_REGS_H
> >>> +#define X86_CPU_USER_REGS_H
> >>> +
> >>> +#define DECL_REG_LOHI(which) union { \
> >>> +    uint64_t r ## which ## x; \
> >>> +    uint32_t e ## which ## x; \
> >>> +    uint16_t which ## x; \
> >>> +    struct { \
> >>> +        uint8_t which ## l; \
> >>> +        uint8_t which ## h; \
> >>> +    }; \
> >>> +}
> >>> +#define DECL_REG_LO8(name) union { \
> >>> +    uint64_t r ## name; \
> >>> +    uint32_t e ## name; \
> >>> +    uint16_t name; \
> >>> +    uint8_t name ## l; \
> >>> +}
> >>> +#define DECL_REG_LO16(name) union { \
> >>> +    uint64_t r ## name; \
> >>> +    uint32_t e ## name; \
> >>> +    uint16_t name; \
> >>> +}
> >>> +#define DECL_REG_HI(num) union { \
> >>> +    uint64_t r ## num; \
> >>> +    uint32_t r ## num ## d; \
> >>> +    uint16_t r ## num ## w; \
> >>> +    uint8_t r ## num ## b; \
> >>> +}
> >> Can we try to avoid repeating these here? The #undef-s in the public 
> >> header are
> >> to keep external consumers' namespaces reasonably tidy. In Xen, since we 
> >> don't
> >> otherwise use identifiers of these names, can't we simply #ifdef-out those
> >> #undef-s, and then not re-introduce the same (less the two underscores) 
> >> here?
> >> Granted we then need to include the public header here, but I think that's 
> >> a
> >> fair price to pay to avoid the redundancy.
> > 
> > Breaking the connection between asm/current.h and public/xen.h is very
> > important IMO.  Right now, the public interface/types/defines are in
> > every TU, and they absolutely shouldn't be.
> 
> Hmm, that's a good point. Nevertheless I wonder if we still couldn't avoid the
> unhelpful redundancy. E.g. by introducing a separate, small public header with
> just these. Which we'd then pull in here as well.
> 
> > Sadly, the compiler isn't happy when including public/xen.h after
> > asm/current.h, hence the dropping of the underscores.
> 
> Even if the ones here are #undef-ed after use?
> 
> > I did have half a mind to expand them fully.  I find them unintuitive,
> > but I also didn't think I'd successfully argue that change in.
> 
> Roger - do you have an opinion here? I like these wrappers, yet then I also
> understand this is code that's pretty unlikely to ever change again. Hence
> fully expanding them is an option.

Hm, I don't have a strong opinion TBH, as I haven't done much work
that required touching those.  I think the proposal is fine, we can
always fully expand later if needed.

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.