Re: Emulating lwarx and stwcx instructions in PowerPc BOOKE e500

[Scott Wood <scottwood@freescale.com>]

On 03/05/2012 02:37 PM, Aashish Mittal wrote:
> Hi
> I'm working on powerpc booke architecture and my project requires me to remove
> read and write privileges on some pages. Due to this any instruction accessing
> these pages traps and i'm trying to emulate the behavior of these instructions.
> 
> I've emulated lwarx and stwcx instruction but i think stwcx is not working
> correctly. The emulation i've written is written below

What is it you're emulating that needs lwarx/stwcx to work?

> case OP_31_XOP_LWARX:
> {
>   ulong ret;
>   ulong addr;
>   int eh = inst & 0x00000001 ;
>   kvm_gva_to_hva(vcpu,ea,&addr);
>   /*lwarx RT RA RB EH*/
>   if(eh = 0)
>       __asm__ __volatile__("lwarx %0,0,%1,0; isync":"=r" (ret) :"r" (addr));
>   else
>      __asm__ __volatile__("lwarx %0,0,%1,1; isync":"=r" (ret) :"r" (addr));
> 	      
>   kvmppc_set_gpr(vcpu,rt,ret);
> }
> 
> case OP_31_XOP_STWCX:
> {
>   ulong tmp;
>   ulong addr;
>   ulong data;
>   kvm_gva_to_hva(vcpu,ea,&addr);
>   kvmppc_read_guest(vcpu,ea,&data,sizeof(data));
>   __asm__ __volatile__("stwcx. %1,0,%2; isync"
>   :"=r" (tmp):"r" (data),"r" (addr):"memory");
> 
> } 
> 
> Here kvm_gva_to_hva function convrets a guest effective address to host virtual
> address .
> 
> void kvm_gva_to_hva(struct kvm_vcpu *vcpu, ulong ea,ulong* hva)
> {
>   gfn_t gfn;
>   gpa_t gpa ;
>   int gtlb_index;
>   int offset;
>   ulong addr;
>   struct kvmppc_vcpu_e500 *vcpu_e500 = to_e500(vcpu);
> 	
>   gtlb_index = kvmppc_mmu_itlb_index(vcpu, ea);
>   gpa = kvmppc_mmu_xlate(vcpu,gtlb_index, ea);
>   gfn = gpa >> PAGE_SHIFT;
>   addr = (ulong)gfn_to_hva(vcpu_e500->vcpu.kvm, gfn);
>   offset = offset_in_page(gpa);
>   
>   *hva = addr + offset;
>   return;
> }
> 
> The guest just hangs once it encounters a stwcx instruction. Does anybody have
> any idea why this is not working and what's wrong about the emulation code.

You're losing the reservation somewhere.  Any lock or atomic operation
along the emulation path will do this.

Even if this didn't happen by accident, we really don't want to leave a
reservation when we return to the guest -- it could have belonged to a
previously running guest operating on shared memory, for example.
Perhaps we should have a dummy stwcx on KVM guest entry code, similar to
the one on interrupt return?

> Also i'm working on linux-3.0-rc4 kernel .

Why are you working on something other than the current code or a stable
release?

-Scott