Re: [RFC v3 21/21] x86/boot/compressed/64: use 1GB pages for mappings

[H. Peter Anvin]

On April 26, 2023 5:08:57 PM PDT, Anthony Yznaga <anthony.yznaga@xxxxxxxxxx> wrote:
>pkram kaslr code can incur multiple page faults when it walks its
>preserved ranges list called via mem_avoid_overlap().  The multiple
>faults can easily end up using up the small number of pages available
>to be allocated for page table pages.
>
>This patch hacks things so that mappings are 1GB which results in the need
>for far fewer page table pages.  As is this breaks AMD SEV-ES which expects
>the mappings to be 2M.  This could possibly be fixed by updating split
>code to split 1GB page if the aren't any other issues with using 1GB
>mappings.
>
>Signed-off-by: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>
>---
> arch/x86/boot/compressed/ident_map_64.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
>diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
>index 321a5011042d..1e02cf6dda3c 100644
>--- a/arch/x86/boot/compressed/ident_map_64.c
>+++ b/arch/x86/boot/compressed/ident_map_64.c
>@@ -95,8 +95,8 @@ void kernel_add_identity_map(unsigned long start, unsigned long end)
> 	int ret;
> 
> 	/* Align boundary to 2M. */
>-	start = round_down(start, PMD_SIZE);
>-	end = round_up(end, PMD_SIZE);
>+	start = round_down(start, PUD_SIZE);
>+	end = round_up(end, PUD_SIZE);
> 	if (start >= end)
> 		return;
> 
>@@ -120,6 +120,7 @@ void initialize_identity_maps(void *rmode)
> 	mapping_info.context = &pgt_data;
> 	mapping_info.page_flag = __PAGE_KERNEL_LARGE_EXEC | sme_me_mask;
> 	mapping_info.kernpg_flag = _KERNPG_TABLE;
>+	mapping_info.direct_gbpages = true;
> 
> 	/*
> 	 * It should be impossible for this not to already be true,
>@@ -365,8 +366,8 @@ void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code)
> 
> 	ghcb_fault = sev_es_check_ghcb_fault(address);
> 
>-	address   &= PMD_MASK;
>-	end        = address + PMD_SIZE;
>+	address   &= PUD_MASK;
>+	end        = address + PUD_SIZE;
> 
> 	/*
> 	 * Check for unexpected error codes. Unexpected are:

Strong NAK: 1G pages are not supported by all 64-bit CPUs, *and* by your own admission breaks things ...