Re: [PATCH] reserve RAM below PHYSICAL_START

H. Peter Anvin
2008-02-29T18:23:09Z

Hi Andrea,

Sorry for the long delay in replying.

I'm trying to grok the use cases for this. In particular, it seems like
a particularly restricted case of wanting to be able to reserve an
arbitrary bit of memory, which seems like it would be more useful (don't
we already have memmap= options for that, anyway?)

In particular, what's the reason for reserving *low* memory? Low memory
(first megabyte) is full of special-use address space which, as the Xen
address space discussion has showed, is nontrivial to tamper with even
if it initially works. If you want a dedicated chunk of mappable PCI
space, it would seem cleaner to have it higher up in the memory map.

Andrea Arcangeli wrote:
> Hello,
>
> this patch allows to prevent linux from using the ram below
> PHYSICAL_START.
>
> The "reserved RAM" can be mapped by virtualization software with to
> create a 1:1 mapping between guest physical (bus) address and host
> physical (bus) address. This will allow pci passthrough with DMA for
> the guest with current production hardware that misses VT-d. The only
> detail to take care of is the ram marked "reserved RAM failed". The
> virtualization software must create for the guest an e820 map that
> only includes the "reserved RAM" regions but if the guest touches
> memory with guest physical address in the "reserved RAM failed" ranges
> (linux guest will do that even if the ram isn't present in the e820
> map), it should provide that as ram and map it with a not-ident
> mapping. This should allow any linux kernel to run fine with pci
> passthrough and hopefully any other OS too with all VT enabled
> hardware.
>
> (the virtualization software should do if (pfn_valid(gfn))
> get_page(pfn_to_page(gfn)) instead of get_user_pages and equivalent
> check in the release path)
>
> The trampoline page marked as "reserved RAM failed" can be easily
> relocated near 640k with an incremental patch to avoid an e820 hole at
> 0x6000 if any bootloader or OS gets confused.
>
> The end of the patch are just bugfixes. However the limit of the
> reserved ram is 1G... this can also be relaxed with an incremental
> patch later on if needed (currently 1G is enough). Perhaps this has
> other usages.
>
> Let me know if this can be merged, thanks!
>
> svm ~ # cat /proc/iomem |head -n 20
> 00000000-00000fff : reserved RAM failed
> 00001000-00005fff : reserved RAM
> 00006000-00007fff : reserved RAM failed
> 00008000-0009efff : reserved RAM
> 0009f000-0009ffff : reserved
> 000cd600-000cffff : pnp 00:0d
> 000f0000-000fffff : reserved
> 00100000-0fffffff : reserved RAM
> 10000000-3dedffff : System RAM
> 10000000-10329ab2 : Kernel code
> 10329ab3-104933e7 : Kernel data
> 104f5000-10558e67 : Kernel bss
> 3dee0000-3dee2fff : ACPI Non-volatile Storage
> 3dee3000-3deeffff : ACPI Tables
> 3def0000-3defffff : reserved
> 3dff0000-3ffeffff : pnp 00:0d
> e0000000-efffffff : reserved
> fa000000-fbffffff : PCI Bus #01
> fa000000-fbffffff : 0000:01:05.0
> fda00000-fdbfffff : PCI Bus #01
> svm ~ # hexdump /dev/mem | grep -C2 'cccc cccc cccc cccc'
> 00007e0 0000 0000 0000 0000 0000 0000 0000 0000
> *
> 0001000 cccc cccc cccc cccc cccc cccc cccc cccc
> *
> 0006000 a5a5 a5a5 8ec8 8ed8 8ec0 66d0 06c7 0000
> --
> *
> 0007ff0 0000 0000 0000 0000 3063 1000 0000 0000
> 0008000 cccc cccc cccc cccc cccc cccc cccc cccc
> *
> 009f000 0002 0000 0000 0000 0000 0000 0000 0000
> --
> 00fffe0 6000 3c03 45e7 0184 0500 0082 01c0 0223
> 00ffff0 5bea 00e0 31f0 2f32 3931 302f 0037 12fc
> 0100000 cccc cccc cccc cccc cccc cccc cccc cccc
> *
> 10000000 8d48 f92d ffff 48ff ed81 0000 1000 8948
> ^C
> svm ~ #
>
> Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1109,8 +1109,36 @@ config CRASH_DUMP
> (CONFIG_RELOCATABLE=y).
> For more details see Documentation/kdump/kdump.txt
>
> +config RESERVE_PHYSICAL_START
> + bool "Reserve all RAM below PHYSICAL_START (EXPERIMENTAL)"
> + depends on !RELOCATABLE && X86_64
> + help
> + This makes the kernel use only RAM above __PHYSICAL_START.
> + All memory below __PHYSICAL_START will be left unused and
> + marked as "reserved RAM" in /proc/iomem. The few special
> + pages that can't be relocated at addresses above
> + __PHYSICAL_START and that can't be guaranteed to be unused
> + by the running kernel, will be marked "reserved RAM failed"
> + in /proc/iomem. Those may or may be not used by the kernel
> + (for example smp trampoline pages would only be used if
> + CPU hotplug is enabled).
> +
> + The "reserved RAM" can be mapped by virtualization software
> + with /dev/mem to create a 1:1 mapping between guest physical
> + (bus) address and host physical (bus) address. This will
> + allow pci passthrough with DMA for the guest using the ram
> + with the 1:1 mapping. The only detail to take care of is the
> + ram marked "reserved RAM failed". The virtualization
> + software must create for the guest an e820 map that only
> + includes the "reserved RAM" regions but if the guest touches
> + memory with guest physical address in the "reserved RAM
> + failed" ranges (linux guest will do that even if the ram
> + isn't present in the e820 map), it should provide that as
> + ram and map it with a non linear mapping. This should allow
> + any linux kernel to run fine and hopefully any other OS too.
> +
> config PHYSICAL_START
> - hex "Physical address where the kernel is loaded" if (EMBEDDED || CRASH_DUMP)
> + hex "Physical address where the kernel is loaded" if (EMBEDDED || CRASH_DUMP || RESERVE_PHYSICAL_START)
> default "0x1000000" if X86_NUMAQ
> default "0x200000" if X86_64
> default "0x100000"
> diff --git a/arch/x86/kernel/e820_64.c b/arch/x86/kernel/e820_64.c
> --- a/arch/x86/kernel/e820_64.c
> +++ b/arch/x86/kernel/e820_64.c
> @@ -91,6 +91,11 @@ void __init early_res_to_bootmem(void)
> printk(KERN_INFO "early res: %d [%lx-%lx] %s\n", i,
> r->start, r->end - 1, r->name);
> reserve_bootmem_generic(r->start, r->end - r->start);
> +#ifdef CONFIG_RESERVE_PHYSICAL_START
> + if (r->start < __PHYSICAL_START)
> + add_memory_region(r->start, r->end - r->start,
> + E820_RESERVED_RAM_FAILED);
> +#endif
> }
> }
>
> @@ -231,6 +236,10 @@ void __init e820_reserve_resources(struc
> struct resource *data_resource, struct resource *bss_resource)
> {
> int i;
> +#ifdef CONFIG_RESERVE_PHYSICAL_START
> + /* solve E820_RESERVED_RAM vs E820_RESERVED_RAM_FAILED conflicts */
> + update_e820();
> +#endif
> for (i = 0; i < e820.nr_map; i++) {
> struct resource *res;
> res = alloc_bootmem_low(sizeof(struct resource));
> @@ -238,6 +247,16 @@ void __init e820_reserve_resources(struc
> case E820_RAM: res->name = "System RAM"; break;
> case E820_ACPI: res->name = "ACPI Tables"; break;
> case E820_NVS: res->name = "ACPI Non-volatile Storage"; break;
> +#ifdef CONFIG_RESERVE_PHYSICAL_START
> + case E820_RESERVED_RAM_FAILED:
> + res->name = "reserved RAM failed";
> + break;
> + case E820_RESERVED_RAM:
> + memset(__va(e820.map[i].addr),
> + POISON_FREE_INITMEM, e820.map[i].size);
> + res->name = "reserved RAM";
> + break;
> +#endif
> default: res->name = "reserved";
> }
> res->start = e820.map[i].addr;
> @@ -409,6 +428,12 @@ static void __init e820_print_map(char *
> break;
> case E820_NVS:
> printk(KERN_CONT "(ACPI NVS)\n");
> + break;
> + case E820_RESERVED_RAM:
> + printk(KERN_CONT "(reserved RAM)\n");
> + break;
> + case E820_RESERVED_RAM_FAILED:
> + printk(KERN_CONT "(reserved RAM failed)\n");
> break;
> default:
> printk(KERN_CONT "type %u\n", e820.map[i].type);
> @@ -639,9 +664,31 @@ static int __init copy_e820_map(struct e
> unsigned long end = start + size;
> unsigned long type = biosmap->type;
>
> +#ifdef CONFIG_RESERVE_PHYSICAL_START
> + /* make space for two more low-prio types */
> + type += 2;
> +#endif
> +
> /* Overflow in 64 bits? Ignore the memory map. */
> if (start > end)
> return -1;
> +
> +#ifdef CONFIG_RESERVE_PHYSICAL_START
> + if (type == E820_RAM) {
> + if (end <= __PHYSICAL_START) {
> + add_memory_region(start, size,
> + E820_RESERVED_RAM);
> + continue;
> + }
> + if (start < __PHYSICAL_START) {
> + add_memory_region(start,
> + __PHYSICAL_START-start,
> + E820_RESERVED_RAM);
> + size -= __PHYSICAL_START-start;
> + start = __PHYSICAL_START;
> + }
> + }
> +#endif
>
> add_memory_region(start, size, type);
> } while (biosmap++, --nr_map);
> diff --git a/include/asm-x86/e820.h b/include/asm-x86/e820.h
> --- a/include/asm-x86/e820.h
> +++ b/include/asm-x86/e820.h
> @@ -4,10 +4,19 @@
> #define E820MAX 128 /* number of entries in E820MAP */
> #define E820NR 0x1e8 /* # entries in E820MAP */
>
> +#ifdef CONFIG_RESERVE_PHYSICAL_START
> +#define E820_RESERVED_RAM 1
> +#define E820_RESERVED_RAM_FAILED 2
> +#define E820_RAM 3
> +#define E820_RESERVED 4
> +#define E820_ACPI 5
> +#define E820_NVS 6
> +#else
> #define E820_RAM 1
> #define E820_RESERVED 2
> #define E820_ACPI 3
> #define E820_NVS 4
> +#endif
>
> #ifndef __ASSEMBLY__
> struct e820entry {
> diff --git a/include/asm-x86/page_64.h b/include/asm-x86/page_64.h
> --- a/include/asm-x86/page_64.h
> +++ b/include/asm-x86/page_64.h
> @@ -29,6 +29,7 @@
> #define __PAGE_OFFSET _AC(0xffff810000000000, UL)
>
> #define __PHYSICAL_START CONFIG_PHYSICAL_START
> +#define __PHYSICAL_OFFSET (__PHYSICAL_START-0x200000)
> #define __KERNEL_ALIGN 0x200000
>
> /*
> @@ -47,7 +48,7 @@
> #define __PHYSICAL_MASK_SHIFT 46
> #define __VIRTUAL_MASK_SHIFT 48
>
> -#define KERNEL_TEXT_SIZE (40*1024*1024)
> +#define KERNEL_TEXT_SIZE (40*1024*1024+__PHYSICAL_OFFSET)
> #define KERNEL_TEXT_START _AC(0xffffffff80000000, UL)
>
> #ifndef __ASSEMBLY__
> diff --git a/include/asm-x86/pgtable_64.h b/include/asm-x86/pgtable_64.h
> --- a/include/asm-x86/pgtable_64.h
> +++ b/include/asm-x86/pgtable_64.h
> @@ -140,7 +140,7 @@ static inline void native_pgd_clear(pgd_
> #define VMALLOC_START _AC(0xffffc20000000000, UL)
> #define VMALLOC_END _AC(0xffffe1ffffffffff, UL)
> #define VMEMMAP_START _AC(0xffffe20000000000, UL)
> -#define MODULES_VADDR _AC(0xffffffff88000000, UL)
> +#define MODULES_VADDR (0xffffffff88000000UL+__PHYSICAL_OFFSET)
> #define MODULES_END _AC(0xfffffffffff00000, UL)
> #define MODULES_LEN (MODULES_END - MODULES_VADDR)
>
>
> --
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