Re: [PATCH 14/22] x86/traps: Extend struct cpu_user_regs/cpu_info with FRED fields

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.08.2025 22:23, Andrew Cooper wrote:
> @@ -42,17 +46,76 @@ struct cpu_user_regs
>       */
>  
>      union { uint64_t rip;    uint32_t eip;    uint16_t ip; };
> -    uint16_t cs, _pad0[1];
> -    uint8_t  saved_upcall_mask; /* PV (v)rflags.IF == !saved_upcall_mask */
> -    uint8_t  _pad1[3];
> +    union {
> +        struct {
> +            uint16_t      cs;
> +            unsigned long :16;
> +            uint8_t       saved_upcall_mask; /* PV (v)rflags.IF == 
> !saved_upcall_mask */

Would this better be reproduced ...

> +        };
> +        unsigned long     csx;
> +        struct {
> +            /*
> +             * Bits 0 thru 31 control ERET{U,S} behaviour, and is state of 
> the
> +             * interrupted context.
> +             */
> +            uint16_t      cs;
> +            unsigned int  sl:2;      /* Stack Level */
> +            bool          wfe:1;     /* Wait-for-ENDBRANCH state */

... here as well, just like you reproduce "cs"?

> +        } fred_cs;
> +    };
>      union { uint64_t rflags; uint32_t eflags; uint16_t flags; };
>      union { uint64_t rsp;    uint32_t esp;    uint16_t sp;    uint8_t spl; };
> -    uint16_t ss, _pad2[3];
> +    union {
> +        uint16_t          ss;
> +        unsigned long     ssx;

What use do you foresee for this and "csx"?

> +        struct {
> +            /*
> +             * Bits 0 thru 31 control ERET{U,S} behaviour, and is state about
> +             * the event which occured.
> +             */
> +            uint16_t      ss;
> +            bool          sti:1;     /* Was blocked-by-STI, and not 
> cancelled */
> +            bool          swint:1;   /* Was a SYSCALL/SYSENTER/INT $N */
> +            bool          nmi:1;     /* Was an NMI. */
> +            unsigned long :13;
> +
> +            /*
> +             * Bits 32 thru 63 are ignored by ERET{U,S} and are informative
> +             * only.
> +             */
> +            uint8_t       vector;
> +            unsigned long :8;
> +            unsigned int  type:4;    /* X86_ET_* */
> +            unsigned long :4;
> +            bool          enclave:1; /* Event taken in SGX mode */
> +            bool          lm:1;      /* Was in Long Mode */

The bit indicates 64-bit mode aiui, not long mode (without which FRED isn't even
available).

> --- a/xen/arch/x86/include/asm/current.h
> +++ b/xen/arch/x86/include/asm/current.h
> @@ -38,6 +38,8 @@ struct vcpu;
>  
>  struct cpu_info {
>      struct cpu_user_regs guest_cpu_user_regs;
> +    struct fred_info _fred; /* Only used when FRED is active. */

Any particular need for the leading underscore?

Jan