On 08.07.2025 08:37, Juergen Gross wrote:
> Add a small pool of statically allocated memory pages to be handed out
> for very early page table allocations.
>
> This will make it possible to do virtual allocations e.g. for mapping
> the shared info page.
>
> Signed-off-by: Juergen Gross <jgr...@suse.com>
> ---
> arch/x86/mm.c | 16 +++++++++++++++-
> 1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm.c b/arch/x86/mm.c
> index bdff38fd..3f5c7ea7 100644
> --- a/arch/x86/mm.c
> +++ b/arch/x86/mm.c
> @@ -640,12 +640,17 @@ void change_readonly(bool readonly)
> * return a valid PTE for a given virtual address. If PTE does not exist,
> * allocate page-table pages.
> */
> +#define N_PTS 4
Wouldn't it be prudent to have a comment here mentioning how this number
was derived, i.e. what's known to be covered? (To map the shared info
page I expect you really only need 3? Hence without a comment things may
remain unclear.)
> +static char early_pt[PAGE_SIZE * N_PTS] __attribute__((aligned(PAGE_SIZE)));
Maybe better early_pt[N_PTS][PAGE_SIZE], simplifying the allocation
code below a little?
> +static unsigned long n_early_pt = N_PTS;
unsigned int would do, I expect? With the suggestion above this could
then also use ARRAY_SIZE(), at which point there would be no real need
for N_PTS anymore.
> static int need_pgt_func(unsigned long va, unsigned int lvl, bool is_leaf,
> pgentry_t *pte, void *par)
> {
> pgentry_t **result = par;
> unsigned long pt_mfn;
> unsigned long pt_pfn;
> + unsigned long pt_addr;
> unsigned int idx;
>
> if ( !is_leaf )
> @@ -664,7 +669,16 @@ static int need_pgt_func(unsigned long va, unsigned int
> lvl, bool is_leaf,
> }
>
> pt_mfn = virt_to_mfn(pte);
> - pt_pfn = virt_to_pfn(alloc_page());
> + if ( n_early_pt )
> + {
> + n_early_pt--;
> + pt_addr = (unsigned long)&early_pt[n_early_pt * PAGE_SIZE];
> + }
> + else
> + {
> + pt_addr = alloc_page();
> + }
The failure pattern when one fails to increase early_pt[] is likely going
to be problematic. Wouldn't it be better to check for failure here?
Jan
