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This is > > expensive when not using SPEC_CTRL, and hence should be avoided as > > much as possible. > > > > When SSBD is not being set from SPEC_CTRL on AMD don't context switch > > at vm{entry,exit} and instead only context switch SSBD when switching > > vCPUs. This has the side effect of running Xen code with the guest > > selection of SSBD, the documentation is updated to note this behavior. > > Also note that then when `ssbd` is selected on the command line guest > > SSBD selection will not have an effect, and the hypervisor will run > > with SSBD unconditionally enabled when not using SPEC_CTRL itself. > > > > This fixes an issue with running C code in a GIF=0 region, that's > > problematic when using UBSAN or other instrumentation techniques. > > > > As a result of no longer running the code to set SSBD in a GIF=0 > > region the locking of amd_set_legacy_ssbd() can be done using normal > > spinlocks, and some more checks can be added to assure it works as > > intended. > > > > Finally it's also worth noticing that since the guest SSBD selection > > is no longer set on vmentry the VIRT_SPEC_MSR handling needs to > > propagate the value to the hardware as part of handling the wrmsr. > > > > Signed-off-by: Roger Pau Monné > > Reviewed-by: Jan Beulich > with one further remark: > > > --- a/xen/arch/x86/hvm/svm/svm.c > > +++ b/xen/arch/x86/hvm/svm/svm.c > > @@ -973,6 +973,16 @@ static void cf_check svm_ctxt_switch_from(struct vcpu *v) > > > > /* Resume use of ISTs now that the host TR is reinstated. */ > > enable_each_ist(idt_tables[cpu]); > > + > > + /* > > + * Clear previous guest selection of SSBD if set. Note that SPEC_CTRL.SSBD > > + * is already cleared by svm_vmexit_spec_ctrl. > > + */ > > + if ( v->arch.msrs->virt_spec_ctrl.raw & SPEC_CTRL_SSBD ) > > + { > > + ASSERT(v->domain->arch.cpuid->extd.virt_ssbd); > > + amd_set_ssbd(false); > > + } > > } > > Is "cleared" in the comment correct when "spec-ctrl=ssbd"? I think "suitably > set" or "cleared/set" or some such would be wanted. This could certainly be > adjusted while committing (if you agree), but I will want to give Andrew some > time anyway before putting it in, to avoid there again being objections after > a change in this area has gone in. Hm, indeed, maybe "already handled" to avoid getting into the set/clear nomenclature. Thanks, Roger.