Re: [PATCH v2 7/7] xen/ppc: mm-radix: Allocate Partition and Process Tables at runtime

From: Jan Beulich <jbeulich@suse.com>
To: Shawn Anastasio <sanastasio@raptorengineering.com>
Cc: Timothy Pearson <tpearson@raptorengineering.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2 7/7] xen/ppc: mm-radix: Allocate Partition and Process Tables at runtime
Date: Thu, 21 Dec 2023 08:06:30 +0100	[thread overview]
Message-ID: <6ba01b31-fc51-4d12-83a2-4754ccfb6339@suse.com> (raw)
In-Reply-To: <f49a4a372a9f82e217fa56ba0dc3068deff32ef5.1702607884.git.sanastasio@raptorengineering.com>

On 15.12.2023 03:44, Shawn Anastasio wrote:
> In the initial mm-radix implementation, the in-memory partition and
> process tables required to configure the MMU were allocated statically
> since the boot allocator was not yet available.
> 
> Now that it is, allocate these tables at runtime and bump the size of
> the Process Table to its maximum supported value (on POWER9). Also bump
> the number of static LVL2/3 PD frames to tolerate cases where the boot
> allocator returns an address outside of the range of the LVL2 frame used
> for Xen.

Don't you need to bump to 4, in case PATB and PRTB end up sufficiently
far apart? Or even further, considering that you may need distinct L2
and L3 for each of Xen, PATB, and PRTB?

However, with there being memory to allocate now, is there a reason you
still reserve (perhaps more than necessary) static memory for the
page tables, rather than allocating those dynamically as well?

> @@ -105,80 +157,43 @@ static void __init setup_initial_mapping(struct lvl1_pd *lvl1,
>          die();
>      }
> 
> +    /* Identity map Xen itself */
>      for ( page_addr = map_start; page_addr < map_end; page_addr += PAGE_SIZE )
>      {
> -        struct lvl2_pd *lvl2;
> -        struct lvl3_pd *lvl3;
> -        struct lvl4_pt *lvl4;
> -        pde_t *pde;
> -        pte_t *pte;
> -
> -        /* Allocate LVL 2 PD if necessary */
> -        pde = pt_entry(lvl1, page_addr);
> -        if ( !pde_is_valid(*pde) )
> -        {
> -            lvl2 = lvl2_pd_pool_alloc();
> -            *pde = paddr_to_pde(__pa(lvl2), PDE_VALID,
> -                                XEN_PT_ENTRIES_LOG2_LVL_2);
> -        }
> -        else
> -            lvl2 = __va(pde_to_paddr(*pde));
> +        unsigned long flags;
> 
> -        /* Allocate LVL 3 PD if necessary */
> -        pde = pt_entry(lvl2, page_addr);
> -        if ( !pde_is_valid(*pde) )
> +        if ( is_kernel_text(page_addr) || is_kernel_inittext(page_addr) )
>          {
> -            lvl3 = lvl3_pd_pool_alloc();
> -            *pde = paddr_to_pde(__pa(lvl3), PDE_VALID,
> -                                XEN_PT_ENTRIES_LOG2_LVL_3);
> +            radix_dprintk("%016lx being marked as TEXT (RX)\n", page_addr);
> +            flags = PTE_XEN_RX;
>          }
> -        else
> -            lvl3 = __va(pde_to_paddr(*pde));
> -
> -        /* Allocate LVL 4 PT if necessary */
> -        pde = pt_entry(lvl3, page_addr);
> -        if ( !pde_is_valid(*pde) )
> -        {
> -            lvl4 = lvl4_pt_pool_alloc();
> -            *pde = paddr_to_pde(__pa(lvl4), PDE_VALID,
> -                                XEN_PT_ENTRIES_LOG2_LVL_4);
> -        }
> -        else
> -            lvl4 = __va(pde_to_paddr(*pde));
> -
> -        /* Finally, create PTE in LVL 4 PT */
> -        pte = pt_entry(lvl4, page_addr);
> -        if ( !pte_is_valid(*pte) )
> +        else if ( is_kernel_rodata(page_addr) )
>          {
> -            unsigned long paddr = (page_addr - map_start) + phys_base;
> -            unsigned long flags;
> -
> -            radix_dprintk("%016lx being mapped to %016lx\n", paddr, page_addr);
> -            if ( is_kernel_text(page_addr) || is_kernel_inittext(page_addr) )
> -            {
> -                radix_dprintk("%016lx being marked as TEXT (RX)\n", page_addr);
> -                flags = PTE_XEN_RX;
> -            }
> -            else if ( is_kernel_rodata(page_addr) )
> -            {
> -                radix_dprintk("%016lx being marked as RODATA (RO)\n", page_addr);
> -                flags = PTE_XEN_RO;
> -            }
> -            else
> -            {
> -                radix_dprintk("%016lx being marked as DEFAULT (RW)\n", page_addr);
> -                flags = PTE_XEN_RW;
> -            }
> -
> -            *pte = paddr_to_pte(paddr, flags);
> -            radix_dprintk("%016lx is the result of PTE map\n",
> -                paddr_to_pte(paddr, flags).pte);
> +            radix_dprintk("%016lx being marked as RODATA (RO)\n", page_addr);
> +            flags = PTE_XEN_RO;
>          }
>          else
>          {
> -            early_printk("BUG: Tried to create PTE for already-mapped page!");
> -            die();
> +            radix_dprintk("%016lx being marked as DEFAULT (RW)\n", page_addr);
> +            flags = PTE_XEN_RW;
>          }
> +
> +        map_page_initial(lvl1, page_addr, (page_addr - map_start) + phys_base, flags);
> +    }
> +
> +    /* Map runtime-allocated PATB, PRTB */
> +    for ( page_addr = (uint64_t)initial_patb;
> +          page_addr < (uint64_t)initial_patb + PATB_SIZE;

While technically not an issue, casting pointers to fixed width types is
generally bogus. page_addr itself would likely better also be of a
different type (unsigned long, uintptr_t, or vaddr_t; the latter only if
that's meant to represent hypervisor virtual addresses, not guest ones).

> +          page_addr += PAGE_SIZE )
> +    {
> +        map_page_initial(lvl1, page_addr, __pa(page_addr), PTE_XEN_RW);
> +    }
> +
> +    for ( page_addr = (uint64_t)initial_prtb;
> +          page_addr < (uint64_t)initial_prtb + PRTB_SIZE;
> +          page_addr += PAGE_SIZE )
> +    {
> +        map_page_initial(lvl1, page_addr, __pa(page_addr), PTE_XEN_RW);
>      }

Just as a remark (you're the maintainer) - in cases like these we generally
prefer to omit the braces.

> @@ -210,6 +225,16 @@ void __init setup_initial_pagetables(void)
>  {
>      struct lvl1_pd *root = lvl1_pd_pool_alloc();
>      unsigned long lpcr;
> +    mfn_t patb_mfn, prtb_mfn;
> +
> +    /* Allocate mfns for in-memory tables using the boot allocator */
> +    prtb_mfn = alloc_boot_pages(PRTB_SIZE / PAGE_SIZE,
> +                                max(1, PRTB_SIZE_LOG2 - PAGE_SHIFT));
> +    patb_mfn = alloc_boot_pages(PATB_SIZE / PAGE_SIZE,
> +                                max(1, PATB_SIZE_LOG2 - PAGE_SHIFT));
> +
> +    initial_patb = __va(mfn_to_maddr(patb_mfn));
> +    initial_prtb = __va(mfn_to_maddr(prtb_mfn));

Overall, what's the plan wrt directmap: Are you meaning to not have one
covering all memory? If you do, I wonder if you wouldn't be better off
mapping memory as you pass it to the boot allocator, such that you
won't need to map things piecemeal like you're doing here.

Jan