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

[Jan Beulich <jbeulich@xxxxxxxx>]

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>

Albeit, besides a few remarks, a suggestion below.

> Jan: Is this what you intended?

Yes.

> 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.

> --- /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.

> +struct cpu_user_regs
> +{
> +    DECL_REG_HI(15);
> +    DECL_REG_HI(14);
> +    DECL_REG_HI(13);
> +    DECL_REG_HI(12);
> +    DECL_REG_LO8(bp);
> +    DECL_REG_LOHI(b);
> +    DECL_REG_HI(11);
> +    DECL_REG_HI(10);
> +    DECL_REG_HI(9);
> +    DECL_REG_HI(8);
> +    DECL_REG_LOHI(a);
> +    DECL_REG_LOHI(c);
> +    DECL_REG_LOHI(d);
> +    DECL_REG_LO8(si);
> +    DECL_REG_LO8(di);
> +    uint32_t error_code;
> +    uint32_t entry_vector;
> +    DECL_REG_LO16(ip);
> +    uint16_t cs, _pad0[1];
> +    uint8_t  saved_upcall_mask;
> +    uint8_t  _pad1[3];
> +    DECL_REG_LO16(flags); /* rflags.IF == !saved_upcall_mask */
> +    DECL_REG_LO8(sp);
> +    uint16_t ss, _pad2[3];
> +    uint16_t es, _pad3[3];
> +    uint16_t ds, _pad4[3];
> +    uint16_t fs, _pad5[3];
> +    uint16_t gs, _pad6[3];

I had to peek ahead at the last patch to figure why you introduce these 4 fields
(plus their padding) here, just to remove them again. Personally I think it 
would
be neater if both were folded; nevertheless I'd like to leave this entirely to
you.

Jan