Re: [PATCH v3 22/75] x86/boot/compressed/64: Add set_page_en/decrypted() helpers

[Borislav Petkov]

On Tue, Apr 28, 2020 at 05:16:32PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@xxxxxxx>
> 
> The functions are needed to map the GHCB for SEV-ES guests. The GHCB is
> used for communication with the hypervisor, so its content must not be
> encrypted. After the GHCB is not needed anymore it must be mapped
> encrypted again so that the running kernel image can safely re-use the
> memory.
> 
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
> ---
>  arch/x86/boot/compressed/ident_map_64.c | 134 ++++++++++++++++++++++++
>  arch/x86/boot/compressed/misc.h         |   2 +
>  2 files changed, 136 insertions(+)

...

> +
> +static int set_clr_page_flags(struct x86_mapping_info *info,
> +			      unsigned long address,
> +			      pteval_t set, pteval_t clr)
> +{
> +	unsigned long scratch, *target;
> +	pgd_t *pgdp = (pgd_t *)top_level_pgt;
> +	p4d_t *p4dp;
> +	pud_t *pudp;
> +	pmd_t *pmdp;
> +	pte_t *ptep, pte;
> +
> +	/*
> +	 * First make sure there is a PMD mapping for 'address'.
> +	 * It should already exist, but keep things generic.
> +	 *
> +	 * To map the page just read from it and fault it in if there is no
> +	 * mapping yet. add_identity_map() can't be called here because that
> +	 * would unconditionally map the address on PMD level, destroying any
> +	 * PTE-level mappings that might already exist.  Also do something
> +	 * useless

You mean something like this?

        asm volatile("mov %[address], %%r9"
                     :: [address] "g" (*(unsigned long *)address)
                     : "r9", "memory");

The "memory" clobber should prevent gcc from optimizing that thing away
and r9 is callee-clobbered.

-- 
Regards/Gruss,
    Boris.

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