| Message ID | 20250106112652.579310-1-andrew.cooper3@citrix.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | x86/boot: Fix zap_low_mappings() to map less of the trampoline | expand |
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@citrix.com> Acked-by: Jan Beulich <jbeulich@suse.com> 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. Jan
diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c index 389d813ebe63..d4e6a9c0a2e0 100644 --- 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); } int setup_compat_arg_xlat(struct vcpu *v)
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@citrix.com> --- CC: Jan Beulich <JBeulich@suse.com> CC: Roger Pau Monné <roger.pau@citrix.com> There's not an obvious candidate for a Fixes tag. --- xen/arch/x86/x86_64/mm.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)