Re: [PATCH RFCv2 12/15] xen/arm: add Persistent Map (PMAP) infrastructure

[Stefano Stabellini <sstabellini@kernel.org>]

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On Sun, 25 Apr 2021, Julien Grall wrote:
> From: Wei Liu <wei.liu2@citrix.com>
> 
> The basic idea is like Persistent Kernel Map (PKMAP) in Linux. We
> pre-populate all the relevant page tables before the system is fully
> set up.
> 
> We will need it on Arm in order to rework the arm64 version of
> xenheap_setup_mappings() as we may need to use pages allocated from
> the boot allocator before they are effectively mapped.
> 
> This infrastructure is not lock-protected therefore can only be used
> before smpboot. After smpboot, map_domain_page() has to be used.
> 
> This is based on the x86 version [1] that was originally implemented
> by Wei Liu.
> 
> Take the opportunity to switch the parameter attr from unsigned to
> unsigned int.
> 
> [1] <e92da4ad6015b6089737fcccba3ec1d6424649a5.1588278317.git.hongyxia@amazon.com>
> 
> Signed-off-by: Wei Liu <wei.liu2@citrix.com>
> Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
> [julien: Adapted for Arm]
> Signed-off-by: Julien Grall <jgrall@amazon.com>
> 
> ---
>     Changes in v2:
>         - New patch
> 
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Wei Liu <wl@xen.org>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Roger Pau Monné <roger.pau@citrix.com>
> Cc: Hongian Xia <hongyax@amazon.com>
> 
> This is mostly a copy of the PMAP infrastructure currently discussed
> on x86. The only difference is how the page-tables are updated.
> 
> I think we want to consider to provide a common infrastructure. But
> I haven't done it yet to gather feedback on the overall series
> first.
> ---
>  xen/arch/arm/Makefile        |   1 +
>  xen/arch/arm/mm.c            |   7 +--
>  xen/arch/arm/pmap.c          | 101 +++++++++++++++++++++++++++++++++++
>  xen/include/asm-arm/config.h |   2 +
>  xen/include/asm-arm/lpae.h   |   8 +++
>  xen/include/asm-arm/pmap.h   |  10 ++++
>  6 files changed, 123 insertions(+), 6 deletions(-)
>  create mode 100644 xen/arch/arm/pmap.c
>  create mode 100644 xen/include/asm-arm/pmap.h
> 
> diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
> index ca75f1040dcc..2196ada24941 100644
> --- a/xen/arch/arm/Makefile
> +++ b/xen/arch/arm/Makefile
> @@ -40,6 +40,7 @@ obj-y += percpu.o
>  obj-y += platform.o
>  obj-y += platform_hypercall.o
>  obj-y += physdev.o
> +obj-y += pmap.init.o
>  obj-y += processor.o
>  obj-y += psci.o
>  obj-y += setup.o
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index d090fdfd5994..5e713b599611 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -288,12 +288,7 @@ void dump_hyp_walk(vaddr_t addr)
>      dump_pt_walk(ttbr, addr, HYP_PT_ROOT_LEVEL, 1);
>  }
>  
> -/*
> - * Standard entry type that we'll use to build Xen's own pagetables.
> - * We put the same permissions at every level, because they're ignored
> - * by the walker in non-leaf entries.
> - */
> -static inline lpae_t mfn_to_xen_entry(mfn_t mfn, unsigned attr)
> +lpae_t mfn_to_xen_entry(mfn_t mfn, unsigned int attr)
>  {
>      lpae_t e = (lpae_t) {
>          .pt = {
> diff --git a/xen/arch/arm/pmap.c b/xen/arch/arm/pmap.c
> new file mode 100644
> index 000000000000..702b1bde982d
> --- /dev/null
> +++ b/xen/arch/arm/pmap.c
> @@ -0,0 +1,101 @@
> +#include <xen/init.h>
> +#include <xen/mm.h>
> +
> +#include <asm/bitops.h>
> +#include <asm/flushtlb.h>
> +#include <asm/pmap.h>
> +
> +/*
> + * To be able to use FIXMAP_PMAP_BEGIN.
> + * XXX: move fixmap definition in a separate header
> + */

yes please :-)


> +#include <xen/acpi.h>
> +
> +/*
> + * Simple mapping infrastructure to map / unmap pages in fixed map.
> + * This is used to set up the page table for mapcache, which is used
> + * by map domain page infrastructure.
> + *
> + * This structure is not protected by any locks, so it must not be used after
> + * smp bring-up.
> + */
> +
> +/* Bitmap to track which slot is used */
> +static unsigned long __initdata inuse;
> +
> +/* XXX: Find an header to declare it */

yep


> +extern lpae_t xen_fixmap[LPAE_ENTRIES];
> +
> +void *__init pmap_map(mfn_t mfn)
> +{
> +    unsigned long flags;
> +    unsigned int idx;
> +    vaddr_t linear;
> +    unsigned int slot;
> +    lpae_t *entry, pte;
> +
> +    BUILD_BUG_ON(sizeof(inuse) * BITS_PER_LONG < NUM_FIX_PMAP);
> +
> +    ASSERT(system_state < SYS_STATE_smp_boot);

One small concern here is that we have been using SYS_STATE_early_boot
to switch implementation of things like xen_map_table. Between
SYS_STATE_early_boot and SYS_STATE_smp_boot there is SYS_STATE_boot.

I guess I am wondering if instead of three potentially different mapping
functions (<= SYS_STATE_early_boot, < SYS_STATE_smp_boot, >=
SYS_STATE_smp_boot) we can get away with only two?



> +    local_irq_save(flags);
> +
> +    idx = find_first_zero_bit(&inuse, NUM_FIX_PMAP);
> +    if ( idx == NUM_FIX_PMAP )
> +        panic("Out of PMAP slots\n");
> +
> +    __set_bit(idx, &inuse);
> +
> +    slot = idx + FIXMAP_PMAP_BEGIN;
> +    ASSERT(slot >= FIXMAP_PMAP_BEGIN && slot <= FIXMAP_PMAP_END);
> +
> +    linear = FIXMAP_ADDR(slot);
> +    /*
> +     * We cannot use set_fixmap() here. We use PMAP when there is no direct map,
> +     * so map_pages_to_xen() called by set_fixmap() needs to map pages on
> +     * demand, which then calls pmap() again, resulting in a loop. Modify the
> +     * PTEs directly instead. The same is true for pmap_unmap().
> +     */
> +    entry = &xen_fixmap[third_table_offset(linear)];
> +
> +    ASSERT(!lpae_is_valid(*entry));
> +
> +    pte = mfn_to_xen_entry(mfn, PAGE_HYPERVISOR_RW);
> +    pte.pt.table = 1;
> +    write_pte(entry, pte);
> +
> +    local_irq_restore(flags);
> +
> +    return (void *)linear;
> +}
> +
> +void __init pmap_unmap(const void *p)
> +{
> +    unsigned long flags;
> +    unsigned int idx;
> +    lpae_t *entry;
> +    lpae_t pte = { 0 };
> +    unsigned int slot = third_table_offset((vaddr_t)p);
> +
> +    ASSERT(system_state < SYS_STATE_smp_boot);
> +    ASSERT(slot >= FIXMAP_PMAP_BEGIN && slot <= FIXMAP_PMAP_END);
> +
> +    idx = slot - FIXMAP_PMAP_BEGIN;
> +    local_irq_save(flags);
> +
> +    __clear_bit(idx, &inuse);
> +    entry = &xen_fixmap[third_table_offset((vaddr_t)p)];
> +    write_pte(entry, pte);
> +    flush_xen_tlb_range_va_local((vaddr_t)p, PAGE_SIZE);
> +
> +    local_irq_restore(flags);
> +}
> +
> +/*
> + * Local variables:
> + * mode: C
> + * c-file-style: "BSD"
> + * c-basic-offset: 4
> + * indent-tabs-mode: nil
> + * End:
> + */
> diff --git a/xen/include/asm-arm/config.h b/xen/include/asm-arm/config.h
> index 85d4a510ce8a..35050855b6e1 100644
> --- a/xen/include/asm-arm/config.h
> +++ b/xen/include/asm-arm/config.h
> @@ -180,6 +180,8 @@
>  #define FIXMAP_MISC     1  /* Ephemeral mappings of hardware */
>  #define FIXMAP_ACPI_BEGIN  2  /* Start mappings of ACPI tables */
>  #define FIXMAP_ACPI_END    (FIXMAP_ACPI_BEGIN + NUM_FIXMAP_ACPI_PAGES - 1)  /* End mappings of ACPI tables */
> +#define FIXMAP_PMAP_BEGIN (FIXMAP_ACPI_END + 1)
> +#define FIXMAP_PMAP_END (FIXMAP_PMAP_BEGIN + NUM_FIX_PMAP - 1)
>  
>  #define NR_hypercalls 64
>  
> diff --git a/xen/include/asm-arm/lpae.h b/xen/include/asm-arm/lpae.h
> index 310f5225e056..81fd482ab2ce 100644
> --- a/xen/include/asm-arm/lpae.h
> +++ b/xen/include/asm-arm/lpae.h
> @@ -4,6 +4,7 @@
>  #ifndef __ASSEMBLY__
>  
>  #include <xen/page-defs.h>
> +#include <xen/mm-frame.h>
>  
>  /*
>   * WARNING!  Unlike the x86 pagetable code, where l1 is the lowest level and
> @@ -168,6 +169,13 @@ static inline bool lpae_is_superpage(lpae_t pte, unsigned int level)
>          third_table_offset(addr)            \
>      }
>  
> +/*
> + * Standard entry type that we'll use to build Xen's own pagetables.
> + * We put the same permissions at every level, because they're ignored
> + * by the walker in non-leaf entries.
> + */
> +lpae_t mfn_to_xen_entry(mfn_t mfn, unsigned int attr);
> +
>  #endif /* __ASSEMBLY__ */
>  
>  /*
> diff --git a/xen/include/asm-arm/pmap.h b/xen/include/asm-arm/pmap.h
> new file mode 100644
> index 000000000000..8e1dce93f8e4
> --- /dev/null
> +++ b/xen/include/asm-arm/pmap.h
> @@ -0,0 +1,10 @@
> +#ifndef __ASM_PMAP_H__
> +#define __ARM_PMAP_H__

ASM/ARM


> +/* Large enough for mapping 5 levels of page tables with some headroom */
> +#define NUM_FIX_PMAP 8
> +
> +void *pmap_map(mfn_t mfn);
> +void pmap_unmap(const void *p);

> +#endif	/* __ASM_PMAP_H__ */