On 02.03.2026 16:20, Andrew Cooper wrote: > On 27/02/2026 11:16 pm, Andrew Cooper wrote: >> Lets just say this took an unreasoanble amount of time and effort to track >> down, when trying to move traps_init() earlier during boot. >> >> When the SYSCALL linkage MSRs are not configured ahead of _svm_cpu_up() on >> the >> BSP, the first context switch into PV uses svm_load_segs() and clobbers the >> later-set-up linkage with the 0's cached here, causing hypercalls issues by >> the PV guest to enter at 0 in supervisor mode on the user stack. >> >> Signed-off-by: Andrew Cooper <[email protected]> >> --- >> CC: Jan Beulich <[email protected]> >> CC: Roger Pau Monné <[email protected]> >> >> v4: >> * New >> >> It occurs to me that it's not actually 0's we cache here. It's whatever >> context was left from prior to Xen. We still don't reliably clean unused >> MSRs.
Actually, with this, ... >> --- a/xen/arch/x86/hvm/svm/svm.c >> +++ b/xen/arch/x86/hvm/svm/svm.c >> @@ -35,6 +35,7 @@ >> #include <asm/p2m.h> >> #include <asm/paging.h> >> #include <asm/processor.h> >> +#include <asm/traps.h> >> #include <asm/vm_event.h> >> #include <asm/x86_emulate.h> >> >> @@ -1581,6 +1582,21 @@ static int _svm_cpu_up(bool bsp) >> /* Initialize OSVW bits to be used by guests */ >> svm_host_osvw_init(); >> >> + /* >> + * VMSAVE writes out the current full FS, GS, LDTR and TR segments, and >> + * the GS_SHADOW, SYSENTER and SYSCALL linkage MSRs. >> + * >> + * The segment data gets modified by the svm_load_segs() optimisation >> for >> + * PV context switches, but all values get reloaded at that point, as >> well >> + * as during context switch from SVM. >> + * >> + * If PV guests are available (and FRED is not in use), it is critical >> + * that the SYSCALL linkage MSRs been configured at this juncture. >> + */ >> + ASSERT(opt_fred >= 0); /* Confirm that FRED-ness has been resolved */ >> + if ( IS_ENABLED(CONFIG_PV) && !opt_fred ) >> + ASSERT(rdmsr(MSR_LSTAR)); > > It has occurred to me that this is subtly wrong. While FRED doesn't use > LSTAR/SFMASK, it does reuse STAR. > > So this needs to be: > > if ( IS_ENABLED(CONFIG_PV) ) > ASSERT(rdmsr(MSR_STAR)); > > with the include dropped, as the final sentence adjusted to say "even > with FRED". ... if we inherit a non-zero value, is the assertion of much use this way? Jan
