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s390/smp: rework absolute lowcore access
Temporary unsetting of the prefix page in memcpy_absolute() routine poses a risk of executing code path with unexpectedly disabled prefix page. This rework avoids the prefix page uninstalling and disabling of normal and machine check interrupts when accessing the absolute zero memory. Although memcpy_absolute() routine can access the whole memory, it is only used to update the absolute zero lowcore. This rework therefore introduces a new mechanism for the absolute zero lowcore access and scraps memcpy_absolute() routine for good. Instead, an area is reserved in the virtual memory that is used for the absolute lowcore access only. That area holds an array of 8KB virtual mappings - one per CPU. Whenever a CPU is brought online, the corresponding item is mapped to the real address of the previously installed prefix page. The absolute zero lowcore access works like this: a CPU calls the new primitive get_abs_lowcore() to obtain its 8KB mapping as a pointer to the struct lowcore. Virtual address references to that pointer get translated to the real addresses of the prefix page, which in turn gets swapped with the absolute zero memory addresses due to prefixing. Once the pointer is not needed it must be released with put_abs_lowcore() primitive: struct lowcore *abs_lc; unsigned long flags; abs_lc = get_abs_lowcore(&flags); abs_lc->... = ...; put_abs_lowcore(abs_lc, flags); To ensure the described mechanism works large segment- and region- table entries must be avoided for the 8KB mappings. Failure to do so results in usage of Region-Frame Absolute Address (RFAA) or Segment-Frame Absolute Address (SFAA) large page fields. In that case absolute addresses would be used to address the prefix page instead of the real ones and the prefixing would get bypassed. Reviewed-by: Heiko Carstens <hca@linux.ibm.com> Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
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Alexander Gordeev
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Jul 28, 2022
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,17 @@ | ||
| /* SPDX-License-Identifier: GPL-2.0 */ | ||
| #ifndef _ASM_S390_ABS_LOWCORE_H | ||
| #define _ASM_S390_ABS_LOWCORE_H | ||
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| #include <asm/lowcore.h> | ||
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| #define ABS_LOWCORE_MAP_SIZE (NR_CPUS * sizeof(struct lowcore)) | ||
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| extern unsigned long __abs_lowcore; | ||
| extern bool abs_lowcore_mapped; | ||
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| struct lowcore *get_abs_lowcore(unsigned long *flags); | ||
| void put_abs_lowcore(struct lowcore *lc, unsigned long flags); | ||
| int abs_lowcore_map(int cpu, struct lowcore *lc); | ||
| void abs_lowcore_unmap(int cpu); | ||
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| #endif /* _ASM_ABS_S390_LOWCORE_H */ |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,88 @@ | ||
| // SPDX-License-Identifier: GPL-2.0 | ||
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| #include <linux/pgtable.h> | ||
| #include <asm/pgtable.h> | ||
| #include <asm/abs_lowcore.h> | ||
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| #define ABS_LOWCORE_UNMAPPED 1 | ||
| #define ABS_LOWCORE_LAP_ON 2 | ||
| #define ABS_LOWCORE_IRQS_ON 4 | ||
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| unsigned long __bootdata_preserved(__abs_lowcore); | ||
| bool __ro_after_init abs_lowcore_mapped; | ||
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| int abs_lowcore_map(int cpu, struct lowcore *lc) | ||
| { | ||
| unsigned long addr = __abs_lowcore + (cpu * sizeof(struct lowcore)); | ||
| unsigned long phys = __pa(lc); | ||
| int rc, i; | ||
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| for (i = 0; i < LC_PAGES; i++) { | ||
| rc = vmem_map_4k_page(addr, phys, PAGE_KERNEL); | ||
| if (rc) { | ||
| for (--i; i >= 0; i--) { | ||
| addr -= PAGE_SIZE; | ||
| vmem_unmap_4k_page(addr); | ||
| } | ||
| return rc; | ||
| } | ||
| addr += PAGE_SIZE; | ||
| phys += PAGE_SIZE; | ||
| } | ||
| return 0; | ||
| } | ||
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| void abs_lowcore_unmap(int cpu) | ||
| { | ||
| unsigned long addr = __abs_lowcore + (cpu * sizeof(struct lowcore)); | ||
| int i; | ||
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| for (i = 0; i < LC_PAGES; i++) { | ||
| vmem_unmap_4k_page(addr); | ||
| addr += PAGE_SIZE; | ||
| } | ||
| } | ||
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| struct lowcore *get_abs_lowcore(unsigned long *flags) | ||
| { | ||
| unsigned long irq_flags; | ||
| union ctlreg0 cr0; | ||
| int cpu; | ||
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| *flags = 0; | ||
| cpu = get_cpu(); | ||
| if (abs_lowcore_mapped) { | ||
| return ((struct lowcore *)__abs_lowcore) + cpu; | ||
| } else { | ||
| if (cpu != 0) | ||
| panic("Invalid unmapped absolute lowcore access\n"); | ||
| local_irq_save(irq_flags); | ||
| if (!irqs_disabled_flags(irq_flags)) | ||
| *flags |= ABS_LOWCORE_IRQS_ON; | ||
| __ctl_store(cr0.val, 0, 0); | ||
| if (cr0.lap) { | ||
| *flags |= ABS_LOWCORE_LAP_ON; | ||
| __ctl_clear_bit(0, 28); | ||
| } | ||
| *flags |= ABS_LOWCORE_UNMAPPED; | ||
| return lowcore_ptr[0]; | ||
| } | ||
| } | ||
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| void put_abs_lowcore(struct lowcore *lc, unsigned long flags) | ||
| { | ||
| if (abs_lowcore_mapped) { | ||
| if (flags) | ||
| panic("Invalid mapped absolute lowcore release\n"); | ||
| } else { | ||
| if (smp_processor_id() != 0) | ||
| panic("Invalid mapped absolute lowcore access\n"); | ||
| if (!(flags & ABS_LOWCORE_UNMAPPED)) | ||
| panic("Invalid unmapped absolute lowcore release\n"); | ||
| if (flags & ABS_LOWCORE_LAP_ON) | ||
| __ctl_set_bit(0, 28); | ||
| if (flags & ABS_LOWCORE_IRQS_ON) | ||
| local_irq_enable(); | ||
| } | ||
| put_cpu(); | ||
| } |
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