Re: [PATCH v5 5/5] KVM: x86: improve reexecute_instruction

[Marcelo Tosatti]

On Fri, Jan 11, 2013 at 04:18:22AM +0800, Xiao Guangrong wrote:
> On 01/11/2013 03:48 AM, Marcelo Tosatti wrote:
> > On Fri, Jan 11, 2013 at 02:05:33AM +0800, Xiao Guangrong wrote:
> >> On 01/11/2013 01:26 AM, Marcelo Tosatti wrote:
> >>> On Tue, Jan 08, 2013 at 02:38:36PM +0800, Xiao Guangrong wrote:
> >>>> The current reexecute_instruction can not well detect the failed instruction
> >>>> emulation. It allows guest to retry all the instructions except it accesses
> >>>> on error pfn
> >>>>
> >>>> For example, some cases are nested-write-protect - if the page we want to
> >>>> write is used as PDE but it chains to itself. Under this case, we should
> >>>> stop the emulation and report the case to userspace
> >>>>
> >>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> >>>> ---
> >>>>  arch/x86/include/asm/kvm_host.h |    7 +++++++
> >>>>  arch/x86/kvm/paging_tmpl.h      |   27 ++++++++++++++++++++-------
> >>>>  arch/x86/kvm/x86.c              |    8 +++++++-
> >>>>  3 files changed, 34 insertions(+), 8 deletions(-)
> >>>>
> >>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> >>>> index c431b33..d6ab8d2 100644
> >>>> --- a/arch/x86/include/asm/kvm_host.h
> >>>> +++ b/arch/x86/include/asm/kvm_host.h
> >>>> @@ -502,6 +502,13 @@ struct kvm_vcpu_arch {
> >>>>  		u64 msr_val;
> >>>>  		struct gfn_to_hva_cache data;
> >>>>  	} pv_eoi;
> >>>> +
> >>>> +	/*
> >>>> +	 * Indicate whether the access faults on its page table in guest
> >>>> +	 * which is set when fix page fault and used to detect unhandeable
> >>>> +	 * instruction.
> >>>> +	 */
> >>>> +	bool write_fault_to_shadow_pgtable;
> >>>>  };
> >>>>
> >>>>  struct kvm_lpage_info {
> >>>> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> >>>> index 67b390d..df50560 100644
> >>>> --- a/arch/x86/kvm/paging_tmpl.h
> >>>> +++ b/arch/x86/kvm/paging_tmpl.h
> >>>> @@ -497,26 +497,34 @@ out_gpte_changed:
> >>>>   * created when kvm establishes shadow page table that stop kvm using large
> >>>>   * page size. Do it early can avoid unnecessary #PF and emulation.
> >>>>   *
> >>>> + * @write_fault_to_shadow_pgtable will return true if the fault gfn is
> >>>> + * currently used as its page table.
> >>>> + *
> >>>>   * Note: the PDPT page table is not checked for PAE-32 bit guest. It is ok
> >>>>   * since the PDPT is always shadowed, that means, we can not use large page
> >>>>   * size to map the gfn which is used as PDPT.
> >>>>   */
> >>>>  static bool
> >>>>  FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
> >>>> -			      struct guest_walker *walker, int user_fault)
> >>>> +			      struct guest_walker *walker, int user_fault,
> >>>> +			      bool *write_fault_to_shadow_pgtable)
> >>>>  {
> >>>>  	int level;
> >>>>  	gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1);
> >>>> +	bool self_changed = false;
> >>>>
> >>>>  	if (!(walker->pte_access & ACC_WRITE_MASK ||
> >>>>  	      (!is_write_protection(vcpu) && !user_fault)))
> >>>>  		return false;
> >>>>
> >>>> -	for (level = walker->level; level <= walker->max_level; level++)
> >>>> -		if (!((walker->gfn ^ walker->table_gfn[level - 1]) & mask))
> >>>> -			return true;
> >>>> +	for (level = walker->level; level <= walker->max_level; level++) {
> >>>> +		gfn_t gfn = walker->gfn ^ walker->table_gfn[level - 1];
> >>>> +
> >>>> +		self_changed |= !(gfn & mask);
> >>>> +		*write_fault_to_shadow_pgtable |= !gfn;
> >>>> +	}
> >>>>
> >>>> -	return false;
> >>>> +	return self_changed;
> >>>>  }
> >>>>
> >>>>  /*
> >>>> @@ -544,7 +552,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
> >>>>  	int level = PT_PAGE_TABLE_LEVEL;
> >>>>  	int force_pt_level;
> >>>>  	unsigned long mmu_seq;
> >>>> -	bool map_writable;
> >>>> +	bool map_writable, is_self_change_mapping;
> >>>>
> >>>>  	pgprintk("%s: addr %lx err %x\n", __func__, addr, error_code);
> >>>>
> >>>> @@ -572,9 +580,14 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
> >>>>  		return 0;
> >>>>  	}
> >>>>
> >>>> +	vcpu->arch.write_fault_to_shadow_pgtable = false;
> >>>> +
> >>>> +	is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,
> >>>> +	      &walker, user_fault, &vcpu->arch.write_fault_to_shadow_pgtable);
> >>>> +
> >>>>  	if (walker.level >= PT_DIRECTORY_LEVEL)
> >>>>  		force_pt_level = mapping_level_dirty_bitmap(vcpu, walker.gfn)
> >>>> -		   || FNAME(is_self_change_mapping)(vcpu, &walker, user_fault);
> >>>> +		   || is_self_change_mapping;
> >>>>  	else
> >>>>  		force_pt_level = 1;
> >>>>  	if (!force_pt_level) {
> >>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >>>> index 6f13e03..2957012 100644
> >>>> --- a/arch/x86/kvm/x86.c
> >>>> +++ b/arch/x86/kvm/x86.c
> >>>> @@ -4810,7 +4810,13 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t cr2)
> >>>>  	 * guest to let CPU execute the instruction.
> >>>>  	 */
> >>>>  	kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
> >>>> -	return true;
> >>>> +
> >>>> +	/*
> >>>> +	 * If the access faults on its page table, it can not
> >>>> +	 * be fixed by unprotecting shadow page and it should
> >>>> +	 * be reported to userspace.
> >>>> +	 */
> >>>> +	return !vcpu->arch.write_fault_to_shadow_pgtable;
> >>>>  }
> >>>
> >>> This sounds wrong: only reporting emulation failure in case 
> >>> of a write fault to shadow pagetable? 
> >>
> >> We suppose unprotecting target-gfn can avoid emulation, the same
> >> as current code. :(
> > 
> > Current code treats access to non-mapped guest address as indication to
> > exit reporting emulation failure.
> > 
> > The patch above restricts emulation failure reporting to when
> > write_fault_to_shadow_pgtable = true.
> 
> In the patch 4:
> 
> +	/*
> +	 * If the instruction failed on the error pfn, it can not be fixed,
> +	 * report the error to userspace.
> +	 */
> +	if (is_error_noslot_pfn(pfn))
> +		return false;
> +
> +	kvm_release_pfn_clean(pfn);
> 
> That means, two cases can cause failure fail:
> 
> 1): access on non-mapped guest address (The same as the current code)
> 2): !vcpu->arch.write_fault_to_shadow_pgtable (The new case added in this patch)
> 
> Hmm, or i missed something?

No, i did. Its correct.

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