Re: [PATCH v2 4/8] xen/riscv: setup fixmap mapping

[Oleksii <oleksii.kurochko@gmail.com>]

Hi Julien,

On Sun, 2024-07-21 at 09:46 +0100, Julien Grall wrote:
> Hi Oleksii,
> 
> On 12/07/2024 17:22, Oleksii Kurochko wrote:
> > Introduce a function to set up fixmap mappings and L0 page
> > table for fixmap.
> > 
> > Additionally, defines were introduced in riscv/config.h to
> > calculate the FIXMAP_BASE address.
> > This involved introducing BOOT_FDT_VIRT_{START, SIZE} and
> > XEN_SIZE, XEN_VIRT_END.
> > 
> > Also, the check of Xen size was updated in the riscv/lds.S
> > script to use XEN_SIZE instead of a hardcoded constant.
> > 
> > Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
> > ---
> > Changes in V2:
> >   - newly introduced patch
> > ---
> >   xen/arch/riscv/include/asm/config.h |  9 ++++++
> >   xen/arch/riscv/include/asm/fixmap.h | 48
> > +++++++++++++++++++++++++++++
> >   xen/arch/riscv/include/asm/mm.h     |  2 ++
> >   xen/arch/riscv/include/asm/page.h   |  7 +++++
> >   xen/arch/riscv/mm.c                 | 35 +++++++++++++++++++++
> >   xen/arch/riscv/setup.c              |  2 ++
> >   xen/arch/riscv/xen.lds.S            |  2 +-
> >   7 files changed, 104 insertions(+), 1 deletion(-)
> >   create mode 100644 xen/arch/riscv/include/asm/fixmap.h
> > 
> > diff --git a/xen/arch/riscv/include/asm/config.h
> > b/xen/arch/riscv/include/asm/config.h
> > index 50583aafdc..3275477c17 100644
> > --- a/xen/arch/riscv/include/asm/config.h
> > +++ b/xen/arch/riscv/include/asm/config.h
> > @@ -74,11 +74,20 @@
> >   #error "unsupported RV_STAGE1_MODE"
> >   #endif
> >   
> > +#define XEN_SIZE                MB(2)
> 
> NIT: I would name it XEN_VIRT_SIZE to be consistent with the
> start/end.
> 
> > +#define XEN_VIRT_END            (XEN_VIRT_START + XEN_SIZE)
> Can we get away with not introducing *_END and just use START, SIZE?
> The 
> reason I am asking is with "end" it is never clear whether it is 
> inclusive or exclusive. For instance, here you use an exclusive range
> but ...
> 
> > +
> > +#define BOOT_FDT_VIRT_START     XEN_VIRT_END
> > +#define BOOT_FDT_VIRT_SIZE      MB(4)
> > +
> >   #define DIRECTMAP_SLOT_END      509
> >   #define DIRECTMAP_SLOT_START    200
> >   #define DIRECTMAP_VIRT_START    SLOTN(DIRECTMAP_SLOT_START)
> >   #define DIRECTMAP_SIZE          (SLOTN(DIRECTMAP_SLOT_END) -
> > SLOTN(DIRECTMAP_SLOT_START))
> >   
> > +#define FIXMAP_BASE             (BOOT_FDT_VIRT_START +
> > BOOT_FDT_VIRT_SIZE)
> > +#define FIXMAP_ADDR(n)          (FIXMAP_BASE + (n) * PAGE_SIZE)
> > +
> >   #define FRAMETABLE_SCALE_FACTOR  (PAGE_SIZE/sizeof(struct
> > page_info))
> >   #define FRAMETABLE_SIZE_IN_SLOTS (((DIRECTMAP_SIZE / SLOTN(1)) /
> > FRAMETABLE_SCALE_FACTOR) + 1)
> >   
> > diff --git a/xen/arch/riscv/include/asm/fixmap.h
> > b/xen/arch/riscv/include/asm/fixmap.h
> > new file mode 100644
> > index 0000000000..fcfb82d69c
> > --- /dev/null
> > +++ b/xen/arch/riscv/include/asm/fixmap.h
> > @@ -0,0 +1,48 @@
> > +/*
> > + * fixmap.h: compile-time virtual memory allocation
> > + */
> > +#ifndef __ASM_FIXMAP_H
> > +#define __ASM_FIXMAP_H
> > +
> > +#include <xen/bug.h>
> > +#include <xen/page-size.h>
> > +#include <xen/pmap.h>
> > +
> > +#include <asm/page.h>
> > +
> > +/* Fixmap slots */
> > +#define FIX_PMAP_BEGIN (0) /* Start of PMAP */
> > +#define FIX_PMAP_END (FIX_PMAP_BEGIN + NUM_FIX_PMAP - 1) /* End of
> > PMAP */
> 
> ... here is seems to be inclusive. Furthermore if you had 32-bit
> address 
> space, it is also quite easy to have to create a region right at the
> top 
> of it. So when END is exclusive, it would become 0.
> 
> So on Arm, we decided to start to get rid of "end". I would consider
> to 
> do the same on RISC-V for new functions.
I will refactor the code and get rid of "end".

> 
> > +#define FIX_MISC (FIX_PMAP_END + 1)  /* Ephemeral mappings of
> > hardware */
> 
> Are you going to use this fixmap? If not, then I would consider to 
> remove it.
Yes, it used now in copy_from_paddr():
   /**
    * copy_from_paddr - copy data from a physical address
    * @dst: destination virtual address
    * @paddr: source physical address
    * @len: length to copy
    */
   void __init copy_from_paddr(void *dst, paddr_t paddr, unsigned long
   len)
   {
       void *src = (void *)FIXMAP_ADDR(FIXMAP_MISC);
   
       while (len) {
           unsigned long l, s;
   
           s = paddr & (PAGE_SIZE-1);
           l = min(PAGE_SIZE - s, len);
   
           set_fixmap(FIXMAP_MISC, maddr_to_mfn(paddr),
   PAGE_HYPERVISOR_WC);
           memcpy(dst, src + s, l);
           clear_fixmap(FIXMAP_MISC);
   
           paddr += l;
           dst += l;
           len -= l;
       }
   }

> 
> > +
> > +#define FIX_LAST FIX_MISC
> > +
> > +#define FIXADDR_START FIXMAP_ADDR(0)
> > +#define FIXADDR_TOP FIXMAP_ADDR(FIX_LAST)
> > +
> > +#ifndef __ASSEMBLY__
> > +
> > +/*
> > + * Direct access to xen_fixmap[] should only happen when {set,
> > + * clear}_fixmap() is unusable (e.g. where we would end up to
> > + * recursively call the helpers).
> > + */
> > +extern pte_t xen_fixmap[];
> > +
> > +/* Map a page in a fixmap entry */
> > +extern void set_fixmap(unsigned int map, mfn_t mfn, unsigned int
> > attributes);
> > +/* Remove a mapping from a fixmap entry */
> > +extern void clear_fixmap(unsigned int map);
> 
> Neither of the functions seem to be implemented in this patch. Can
> you 
> clarify what's the plan for them?
You are right, it could be dropped now. But in future this functions
are used for copy_from_paddr(). Look at the code above.


> 
> Also, I know that for x86/arm, we have some function prefixed with 
> extern. But AFAIK, we are trying to get rid of them.
> 
> In any case, I think for RISC-V we need some consistency. For
> instance, 
> here you define with extern but...
> 
> > +
> > +#define fix_to_virt(slot) ((void *)FIXMAP_ADDR(slot))
> > +
> > +static inline unsigned int virt_to_fix(vaddr_t vaddr)
> > +{
> > +    BUG_ON(vaddr >= FIXADDR_TOP || vaddr < FIXADDR_START);
> > +
> > +    return ((vaddr - FIXADDR_START) >> PAGE_SHIFT);
> > +}
> > +
> > +#endif /* __ASSEMBLY__ */
> > +
> > +#endif /* __ASM_FIXMAP_H */
> > diff --git a/xen/arch/riscv/include/asm/mm.h
> > b/xen/arch/riscv/include/asm/mm.h
> > index 25af9e1aaa..a0bdc2bc3a 100644
> > --- a/xen/arch/riscv/include/asm/mm.h
> > +++ b/xen/arch/riscv/include/asm/mm.h
> > @@ -255,4 +255,6 @@ static inline unsigned int
> > arch_get_dma_bitsize(void)
> >       return 32; /* TODO */
> >   }
> >   
> > +void setup_fixmap_mappings(void);
> 
> ... here it is without.
> 
> > +
> >   #endif /* _ASM_RISCV_MM_H */
> > diff --git a/xen/arch/riscv/include/asm/page.h
> > b/xen/arch/riscv/include/asm/page.h
> > index c831e16417..cbbf3656d1 100644
> > --- a/xen/arch/riscv/include/asm/page.h
> > +++ b/xen/arch/riscv/include/asm/page.h
> > @@ -81,6 +81,13 @@ static inline void flush_page_to_ram(unsigned
> > long mfn, bool sync_icache)
> >       BUG_ON("unimplemented");
> >   }
> >   
> > +/* Write a pagetable entry. */
> > +static inline void write_pte(pte_t *p, pte_t pte)
> > +{
> > +    *p = pte;
> > +    asm volatile ("sfence.vma");
> > +}
> > +
> >   #endif /* __ASSEMBLY__ */
> >   
> >   #endif /* _ASM_RISCV_PAGE_H */
> > diff --git a/xen/arch/riscv/mm.c b/xen/arch/riscv/mm.c
> > index 7d09e781bf..d69a174b5d 100644
> > --- a/xen/arch/riscv/mm.c
> > +++ b/xen/arch/riscv/mm.c
> > @@ -49,6 +49,9 @@ stage1_pgtbl_root[PAGETABLE_ENTRIES];
> >   pte_t __section(".bss.page_aligned") __aligned(PAGE_SIZE)
> >   stage1_pgtbl_nonroot[PGTBL_INITIAL_COUNT * PAGETABLE_ENTRIES];
> >   
> > +pte_t __section(".bss.page_aligned") __aligned(PAGE_SIZE)
> > +xen_fixmap[PAGETABLE_ENTRIES];
> 
> Can you add a BUILD_BUG_ON() to check that the number of entries in
> the 
> fixmap will never be above PAGETABLE_ENTRIES?
Sure. What is the best place? Somewhere in setup_fixmap_mappings()?

> 
> > +
> >   #define
> > HANDLE_PGTBL(curr_lvl_num)                                         
> > \
> >       index = pt_index(curr_lvl_num,
> > page_addr);                              \
> >       if ( pte_is_valid(pgtbl[index])
> > )                                       \
> > @@ -191,6 +194,38 @@ static bool __init
> > check_pgtbl_mode_support(struct mmu_desc *mmu_desc,
> >       return is_mode_supported;
> >   }
> >   
> > +void __init setup_fixmap_mappings(void)
> > +{
> > +    pte_t *pte;
> > +    unsigned int i;
> > +
> > +    pte = &stage1_pgtbl_root[pt_index(HYP_PT_ROOT_LEVEL,
> > FIXMAP_ADDR(0))];
> > +
> > +    for ( i = HYP_PT_ROOT_LEVEL - 1; i != 0; i-- )
> 
> I am a little bit confused with the - 1. Is this because you only
> want 
> to map at L1 (I am not sure if this is the correct naming for RISC-
> V)?
Yes, the idea is that I want to stop in L1 ( 2Mb pages ) as this
mapping is already exist and there will not be need to create a new
table. ( what will fail because boot allocator isn't initialized yet
and alloc_boot_pages() will start to alarm because of
BUG_ON(!nr_bootmem_regions) ).

RISC-V also uses word levels, but the order is an opposite to Arm.

> 
> In any case, I think it would be worth a comment.
Sure, I will add it.


> 
> > +    {
> > +        BUG_ON(!pte_is_valid(*pte));
> > +
> > +        pte = (pte_t *)LOAD_TO_LINK(pte_to_paddr(*pte));
> > +        pte = &pte[pt_index(i, FIXMAP_ADDR(0))];
> > +    }
> > +
> > +    BUG_ON( pte_is_valid(*pte) );
> 
> Coding style: BUG_ON(pte_is_valid(*pte));
> 
> > +
> > +    if ( !pte_is_valid(*pte) )
> 
> I am a bit confused with this check. Above, Xen will crash if the PTE
> is 
> valid. So why do we need a runtime check?
You are right, there is no any sense. We should drop it.

> 
> > +    {
> > +        pte_t tmp = paddr_to_pte(LINK_TO_LOAD((unsigned
> > long)&xen_fixmap), PTE_TABLE);
> > +
> > +        write_pte(pte, tmp);
> > +
> > +        printk("(XEN) fixmap is mapped\n");
> > +    }
> > +
> > +    /*
> > +     * We only need the zeroeth table allocated, but not the PTEs
> > set, because
> > +     * set_fixmap() will set them on the fly.
> 
> This function doesn't seem to exists yet (?).
Not yet. It will be introduced later...

~ Oleksii

> 
> > +     */
> > +}
> > +
> >   /*
> >    * setup_initial_pagetables:
> >    *
> > diff --git a/xen/arch/riscv/setup.c b/xen/arch/riscv/setup.c
> > index 4defad68f4..13f0e8c77d 100644
> > --- a/xen/arch/riscv/setup.c
> > +++ b/xen/arch/riscv/setup.c
> > @@ -46,6 +46,8 @@ void __init noreturn start_xen(unsigned long
> > bootcpu_id,
> >       test_macros_from_bug_h();
> >   #endif
> >   
> > +    setup_fixmap_mappings();
> > +
> >       printk("All set up\n");
> >   
> >       for ( ;; )
> > diff --git a/xen/arch/riscv/xen.lds.S b/xen/arch/riscv/xen.lds.S
> > index 070b19d915..63b1dd7bb6 100644
> > --- a/xen/arch/riscv/xen.lds.S
> > +++ b/xen/arch/riscv/xen.lds.S
> > @@ -181,6 +181,6 @@ ASSERT(!SIZEOF(.got.plt),  ".got.plt non-
> > empty")
> >    * Changing the size of Xen binary can require an update of
> >    * PGTBL_INITIAL_COUNT.
> >    */
> > -ASSERT(_end - _start <= MB(2), "Xen too large for early-boot
> > assumptions")
> > +ASSERT(_end - _start <= XEN_SIZE, "Xen too large for early-boot
> > assumptions")
> >   
> >   ASSERT(_ident_end - _ident_start <= IDENT_AREA_SIZE, "identity
> > region is too big");
> 
> Cheers,
>