Re: [PATCH 2/8] x86/IDT: Collect IDT related content idt.h

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/02/2025 8:27 am, Jan Beulich wrote:
> On 24.02.2025 17:05, Andrew Cooper wrote:
>> Logic concerning the IDT is somewhat different to the other system tables, 
>> and
>> in particular ought not to be in asm/processor.h.  Collect it together a new
>> header.
>>
>> While doing so, make a few minor adjustments:
>>
>>  * Make set_ist() use volatile rather than ACCESS_ONCE(), as
>>    _write_gate_lower() already does, removing the need for xen/lib.h.
> While I don't mind this, I'd still like to mention that one of the first 
> things
> I was told when starting to work on Linux was to avoid volatile about 
> everywhere.

Indeed, but that's for "using volatile variables generally".  Here we're
using it very specifically for a single store.

>
>> --- /dev/null
>> +++ b/xen/arch/x86/include/asm/idt.h
>> @@ -0,0 +1,125 @@
>> +/* SPDX-License-Identifier: GPL-2.0-only */
>> +#ifndef X86_ASM_IDT_H
>> +#define X86_ASM_IDT_H
>> +
>> +#include <xen/bug.h>
>> +#include <xen/types.h>
>> +
>> +#include <asm/x86-defns.h>
>> +
>> +#define IST_NONE 0
>> +#define IST_MCE  1
>> +#define IST_NMI  2
>> +#define IST_DB   3
>> +#define IST_DF   4
>> +#define IST_MAX  4
>> +
>> +typedef union {
>> +    struct {
>> +        uint64_t a, b;
>> +    };
>> +    struct {
>> +        uint16_t addr0;
>> +        uint16_t cs;
>> +        uint8_t  ist; /* :3, 5 bits rsvd, but this yields far better code. 
>> */
>> +        uint8_t  type:4, s:1, dpl:2, p:1;
>> +        uint16_t addr1;
>> +        uint32_t addr2;
>> +        /* 32 bits rsvd. */
>> +    };
>> +} idt_entry_t;
>> +
>> +#define IDT_ENTRIES 256
>> +extern idt_entry_t idt_table[];
>> +extern idt_entry_t *idt_tables[];
>> +
>> +/*
>> + * Set the Interrupt Stack Table used by a particular IDT entry.  Typically
>> + * used on a live IDT, so volatile to disuade clever optimisations.
>> + */
>> +static inline void set_ist(volatile idt_entry_t *idt, unsigned int ist)
>> +{
>> +    /* IST is a 3 bit field, 32 bits into the IDT entry. */
>> +    ASSERT(ist <= IST_MAX);
>> +
>> +    idt->ist = ist;
>> +}
>> +
>> +static inline void enable_each_ist(idt_entry_t *idt)
>> +{
>> +    set_ist(&idt[X86_EXC_DF],  IST_DF);
>> +    set_ist(&idt[X86_EXC_NMI], IST_NMI);
>> +    set_ist(&idt[X86_EXC_MC],  IST_MCE);
>> +    set_ist(&idt[X86_EXC_DB],  IST_DB);
>> +}
>> +
>> +static inline void disable_each_ist(idt_entry_t *idt)
>> +{
>> +    set_ist(&idt[X86_EXC_DF],  IST_NONE);
>> +    set_ist(&idt[X86_EXC_NMI], IST_NONE);
>> +    set_ist(&idt[X86_EXC_MC],  IST_NONE);
>> +    set_ist(&idt[X86_EXC_DB],  IST_NONE);
>> +}
>> +
>> +/*
>> + * Write the lower 64 bits of an IDT Entry. This relies on the upper 32
>> + * bits of the address not changing, which is a safe assumption as all
>> + * functions we are likely to load will live inside the 1GB
>> + * code/data/bss address range.
>> + */
>> +static inline void _write_gate_lower(volatile idt_entry_t *gate,
>> +                                     const idt_entry_t *new)
>> +{
>> +    ASSERT(gate->b == new->b);
>> +    gate->a = new->a;
>> +}
> Would this better move down a few lines, immediately ahead of its two
> use sites?
>
>> +#define _set_gate(gate_addr,type,dpl,addr)               \
> Moving this is questionable, as gates aren't limited to the IDT (in
> principle; yes, we don't use call gates ourselves). However, as you
> move it, my minimal request would be to add the missing blanks here.

_set_gate() doesn't survive to the end of the series, which also fixes
the position of _write_gate_lower().


> Beyond that I wonder ...
>
>> +do {                                                     \
>> +    (gate_addr)->a = 0;                                  \
>> +    smp_wmb(); /* disable gate /then/ rewrite */         \
>> +    (gate_addr)->b =                                     \
>> +        ((unsigned long)(addr) >> 32);                   \
>> +    smp_wmb(); /* rewrite /then/ enable gate */          \
>> +    (gate_addr)->a =                                     \
>> +        (((unsigned long)(addr) & 0xFFFF0000UL) << 32) | \
>> +        ((unsigned long)(dpl) << 45) |                   \
>> +        ((unsigned long)(type) << 40) |                  \
>> +        ((unsigned long)(addr) & 0xFFFFUL) |             \
>> +        ((unsigned long)__HYPERVISOR_CS << 16) |         \
>> +        (1UL << 47);                                     \
>> +} while (0)
> ... whether using the other half of the union would allow this to
> become a little more readable. (Then it would also rightfully live
> here, seeing that the union is typedef-ed to idt_entry_t.) This then
> may also extend to ...
>
>> +static inline void _set_gate_lower(idt_entry_t *gate, unsigned long type,
>> +                                   unsigned long dpl, void *addr)
>> +{
>> +    idt_entry_t idte;
>> +    idte.b = gate->b;
>> +    idte.a =
>> +        (((unsigned long)(addr) & 0xFFFF0000UL) << 32) |
>> +        ((unsigned long)(dpl) << 45) |
>> +        ((unsigned long)(type) << 40) |
>> +        ((unsigned long)(addr) & 0xFFFFUL) |
>> +        ((unsigned long)__HYPERVISOR_CS << 16) |
>> +        (1UL << 47);
> ... here and ...
>
>> +    _write_gate_lower(gate, &idte);
>> +}
>> +
>> +/*
>> + * Update the lower half handler of an IDT entry, without changing any other
>> + * configuration.
>> + */
>> +static inline void _update_gate_addr_lower(idt_entry_t *gate, void *addr)
>> +{
>> +    idt_entry_t idte;
>> +    idte.a = gate->a;
>> +
>> +    idte.b = ((unsigned long)(addr) >> 32);
>> +    idte.a &= 0x0000FFFFFFFF0000ULL;
>> +    idte.a |= (((unsigned long)(addr) & 0xFFFF0000UL) << 32) |
>> +        ((unsigned long)(addr) & 0xFFFFUL);
> ... here. Otoh you may have reasons to keep these like they are?

I need to draw the line somewhere on cleanups.  I'm already at 50
patches and I still don't have FRED setup working.

These probably can be cleaned up, but at some later point.

~Andrew