[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] x86/boot: Fix zap_low_mappings() to map less of the trampoline
On Wed, Mar 12, 2025 at 09:31:37AM +0100, Jan Beulich wrote: > On 11.03.2025 21:47, Andrew Cooper wrote: > > On 06/01/2025 11:54 am, Jan Beulich wrote: > >> On 06.01.2025 12:26, Andrew Cooper wrote: > >>> Regular data access into the trampoline is via the directmap. > >>> > >>> As now discussed quite extensively in asm/trampoline.h, the trampoline is > >>> arranged so that only the AP and S3 paths need an identity mapping, and > >>> that > >>> they fit within a single page. > >>> > >>> Right now, PFN_UP(trampoline_end - trampoline_start) is 2, causing more > >>> than > >>> expected of the trampoline to be mapped. Cut it down just the single > >>> page it > >>> ought to be. > >>> > >>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> > >> Acked-by: Jan Beulich <jbeulich@xxxxxxxx> > > > > Thanks. However, > > > >> on the basis that this improves things. However, ... > >> > >>> --- a/xen/arch/x86/x86_64/mm.c > >>> +++ b/xen/arch/x86/x86_64/mm.c > >>> @@ -718,14 +718,16 @@ void __init zap_low_mappings(void) > >>> { > >>> BUG_ON(num_online_cpus() != 1); > >>> > >>> - /* Remove aliased mapping of first 1:1 PML4 entry. */ > >>> + /* Stop using l?_bootmap[] mappings. */ > >>> l4e_write(&idle_pg_table[0], l4e_empty()); > >>> flush_local(FLUSH_TLB_GLOBAL); > >>> > >>> - /* Replace with mapping of the boot trampoline only. */ > >>> + /* > >>> + * Insert an identity mapping of the AP/S3 part of the trampoline, > >>> which > >>> + * is arranged to fit in a single page. > >>> + */ > >>> map_pages_to_xen(trampoline_phys, maddr_to_mfn(trampoline_phys), > >>> - PFN_UP(trampoline_end - trampoline_start), > >>> - __PAGE_HYPERVISOR_RX); > >>> + 1, __PAGE_HYPERVISOR_RX); > >> ... literal numbers like this - however well they are commented - are > >> potentially problematic to locate in case something changes significantly. > >> The 1 here really would want connecting with the .equ establishing > >> wakeup_stack. > > > > how do you propose doing this? > > > > PFN_UP(wakeup_stack - trampoline_start) doesn't have the same obvious > > connection, and it would involve partly undoing 7d73c6f196a5 which hid > > the symbol recently. > > > > While 1 isn't ideal, it is next to a comment explaining what's going on, > > and it's not going to go stale in a silent way... (It's also not liable > > to go stale either.) > > If in > > .equ wakeup_stack, trampoline_start + PAGE_SIZE > > PAGE_SIZE was replaced by a new (in asm/trampoline.h) TRAMPOLINE_PERM_SIZE, > you could use PFN_UP(TRAMPOLINE_PERM_SIZE) here to establish a connection. > > I have to admit I also don't really see why things going stale here would > (a) be unlikely and (b) be guaranteed to not go silently. We just don't > know what we may need to add to the trampoline, sooner or later. Maybe we could introduce trampoline_{ap?,runtime?}_{start,end} that covers this more narrow section of the trampoline code? Thanks, Roger.
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