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SubjectRe: [PATCH, RFC] x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G
"Kirill A. Shutemov" <kirill@shutemov.name> writes:

> On Mon, Oct 09, 2017 at 09:54:53AM -0700, Dave Hansen wrote:
>> On 10/09/2017 09:09 AM, Kirill A. Shutemov wrote:
>> > Apart from trampoline itself we also need place to store top level page
>> > table in lower memory as we don't have a way to load 64-bit value into
>> > CR3 from 32-bit mode. We only really need 8-bytes there as we only use
>> > the very first entry of the page table.
>>
>> Oh, and this is why you have to move "lvl5_pgtable" out of the kernel image?
>
> Right. I initialize the new location of top level page table directly.

So just a quick note. I have a fuzzy memory of people loading their
kernels above 4G physical because they did not have any memory below
4G.

That might be a very specialized case if my memory is correct because
cpu startup has to have a trampoline below 1MB. So I don't know how
that works. But I do seem to remember someone mentioning it.

Is there really no way to switch to 5 level paging other than to drop to
32bit mode and disable paging? The x86 architecture does some very
bizarre things so I can believe it but that seems like a lot of work to
get somewhere.

Eric


>
>> > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
>> > index cefe4958fda9..049a289342bd 100644
>> > --- a/arch/x86/boot/compressed/head_64.S
>> > +++ b/arch/x86/boot/compressed/head_64.S
>> > @@ -288,6 +288,22 @@ ENTRY(startup_64)
>> > leaq boot_stack_end(%rbx), %rsp
>> >
>> > #ifdef CONFIG_X86_5LEVEL
>> > +/*
>> > + * We need trampoline in lower memory switch from 4- to 5-level paging for
>> > + * cases when bootloader put kernel above 4G, but didn't enable 5-level paging
>> > + * for us.
>> > + *
>> > + * Here we use MBR memory to store trampoline code.
>> > + *
>> > + * We also have to have top page table in lower memory as we don't have a way
>> > + * to load 64-bit value into CR3 from 32-bit mode. We only need 8-bytes there
>> > + * as we only use the very first entry of the page table.
>> > + *
>> > + * Here we use 0x7000 as top-level page table.
>> > + */
>> > +#define LVL5_TRAMPOLINE 0x7c00
>> > +#define LVL5_PGTABLE 0x7000
>> > +
>> > /* Preserve RBX across CPUID */
>> > movq %rbx, %r8
>> >
>> > @@ -323,29 +339,37 @@ ENTRY(startup_64)
>> > * long mode would trigger #GP. So we need to switch off long mode
>> > * first.
>> > *
>> > - * NOTE: This is not going to work if bootloader put us above 4G
>> > - * limit.
>> > + * We use trampoline in lower memory to handle situation when
>> > + * bootloader put the kernel image above 4G.
>> > *
>> > * The first step is go into compatibility mode.
>> > */
>> >
>> > - /* Clear additional page table */
>> > - leaq lvl5_pgtable(%rbx), %rdi
>> > - xorq %rax, %rax
>> > - movq $(PAGE_SIZE/8), %rcx
>> > - rep stosq
>> > + /* Copy trampoline code in place */
>> > + movq %rsi, %r9
>> > + leaq lvl5_trampoline(%rip), %rsi
>> > + movq $LVL5_TRAMPOLINE, %rdi
>> > + movq $(lvl5_trampoline_end - lvl5_trampoline), %rcx
>> > + rep movsb
>> > + movq %r9, %rsi
>>
>> This needs to get more heavily commented, like the use of r9 to stash
>> %rsi. Why do you do that, btw? I don't see it getting reused at first
>> glance.
>
> %rsi holds pointer to real_mode_data. It need to be preserved.
>
> I'll add more comments.
>
>> I think it will also be really nice to differentate "lvl5_trampoline"
>> from "LVL5_TRAMPOLINE". Maybe add "src" and "dst" to them or something.
>
> Makes sense. Thanks.
>
>> > /*
>> > - * Setup current CR3 as the first and only entry in a new top level
>> > + * Setup current CR3 as the first and the only entry in a new top level
>> > * page table.
>> > */
>> > movq %cr3, %rdi
>> > leaq 0x7 (%rdi), %rax
>> > - movq %rax, lvl5_pgtable(%rbx)
>> > + movq %rax, LVL5_PGTABLE
>> > +
>> > + /*
>> > + * Load address of lvl5 into RDI.
>> > + * It will be used to return address from trampoline.
>> > + */
>> > + leaq lvl5(%rip), %rdi
>>
>> Is there a reason to do a 'lea' here instead of just shoving the address
>> in directly? Is this a shorter instruction or something?
>
> This code can be loaded anywhere in memory and we need to calculate
> absolute address of the label here.
> AFAIK, "lea <label>(%rip), <register>" is idiomatic way to do this.
>
>> > /* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */
>> > pushq $__KERNEL32_CS
>> > - leaq compatible_mode(%rip), %rax
>> > + movq $LVL5_TRAMPOLINE, %rax
>> > pushq %rax
>> > lretq
>> > lvl5:
>> > @@ -488,9 +512,9 @@ relocated:
>> > */
>> > jmp *%rax
>> >
>> > - .code32
>> > #ifdef CONFIG_X86_5LEVEL
>> > -compatible_mode:
>> > + .code32
>> > +lvl5_trampoline:
>> > /* Setup data and stack segments */
>> > movl $__KERNEL_DS, %eax
>> > movl %eax, %ds
>> > @@ -502,7 +526,7 @@ compatible_mode:
>> > movl %eax, %cr0
>> >
>> > /* Point CR3 to 5-level paging */
>> > - leal lvl5_pgtable(%ebx), %eax
>> > + movl $LVL5_PGTABLE, %eax
>> > movl %eax, %cr3
>> >
>> > /* Enable PAE and LA57 mode */
>> > @@ -510,14 +534,9 @@ compatible_mode:
>> > orl $(X86_CR4_PAE | X86_CR4_LA57), %eax
>> > movl %eax, %cr4
>> >
>> > - /* Calculate address we are running at */
>> > - call 1f
>> > -1: popl %edi
>> > - subl $1b, %edi
>> > -
>> > /* Prepare stack for far return to Long Mode */
>> > pushl $__KERNEL_CS
>> > - leal lvl5(%edi), %eax
>> > + movl $(lvl5_enabled - lvl5_trampoline + LVL5_TRAMPOLINE), %eax
>>
>> This loads the trampoline address of "lvl5_enabled", right? That'd be
>> handy to spell out explicitly.
>
> Yep, will do.

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