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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

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@linux.vnet.ibm.com>
>> ---
>>  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. :(

> 
> The current pattern is sane:
> 
> if (condition_1 which allows reexecution is true)
> 	return true;
> 
> if (condition_2 which allows reexecution is true)
> 	return true;
> ...
> 	return false;

Unfortunately, the current code reports failure only when the access
fault on error pfn:

        pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
        if (!is_error_pfn(pfn)) {
                kvm_release_pfn_clean(pfn);
                return true;
        }

        return false;

All !is_rror_pfn returns true.