Hi all,I understand that in order to improve performance by reducing vm exits, kvm uses one shared page between kvm and guest. Now, guest tells the host to map the magic page to '-4096'.
How does kvm make sure that guest will not use this address ( -4096) for other purpose?
Then, guest itself patches its kernel.I am trying to patch guest from host. So far, I have tried to patch MFMSR instruction. Patching is done when kvm tries to emulate MFMSR. Mechanism used to generate new instruction is same as used by guest in "arch/powerpc/kernel/kvm.c"
--- a/arch/powerpc/kvm/book3s_emulate.c +++ b/arch/powerpc/kvm/book3s_emulate.c@@ -88,6 +88,23 @@ int kvmppc_core_emulate_op(struct kvm_run *run, struct kvm_vcpu *vcpu,
case OP_31_XOP_MFMSR:
kvmppc_set_gpr(vcpu, get_rt(inst),
vcpu->arch.shared->msr);
+
+ pc = kvmppc_get_pc(vcpu);
+
+ //if pte.may_write==false then, kvmppc_st will
not work
+ kvmppc_xlate(vcpu,pc,false, &pte); + pte.may_write=true; + + //Generate new instruction+ addr= (-4096L) + offsetof(struct kvm_vcpu_arch_shared, msr);
+ u32 rt1 = inst & 0x03e00000;+ u32 new_inst=0x80000000 | rt1 | ((addr + 4) & 0x0000fffc);
++ //patch only if magic page is mapped; try patching only for 0x7c0000a6 inst + if (vcpu->arch.magic_page_pa==0xfffff000 && inst==0x7c0000a6) { + kvmppc_st(vcpu,&pc,sizeof(u32),&new_inst,true);
+ }
+
break;
case OP_31_XOP_MTMSRD:
{
But, the problem is guest hangs during boot. I have attached the
screenshot of the guest. I am not able to figure out the problem. Is
there something wrong with the approach?
I have checked that new_inst generated corresponding to 0x7c0000a6 instruction matches with the instruction that is generated when guest patches itself.
Thanks, Dushyant
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