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. --- xen/arch/x86/hvm/svm/svm.c | 16 ++++++++++++++++ xen/arch/x86/setup.c | 2 +- 2 files changed, 17 insertions(+), 1 deletion(-) diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c index 18ba837738c6..f1e02d919cae 100644 --- 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)); + svm_vmsave_pa(per_cpu(host_vmcb, cpu)); return 0; diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c index 27c63d1d97c9..675de3a649ea 100644 --- a/xen/arch/x86/setup.c +++ b/xen/arch/x86/setup.c @@ -2078,7 +2078,7 @@ void asmlinkage __init noreturn __start_xen(void) &this_cpu(stubs).mfn); BUG_ON(!this_cpu(stubs.addr)); - traps_init(); /* Needs stubs allocated. */ + traps_init(); /* Needs stubs allocated, must be before presmp_initcalls. */ cpu_init(); -- 2.39.5
