On Thu, Dec 13, 2012 at 03:23:26AM +0800, Xiao Guangrong wrote: > On 12/12/2012 08:57 AM, Marcelo Tosatti wrote: > > On Mon, Dec 10, 2012 at 05:13:03PM +0800, Xiao Guangrong wrote: > >> We have two issues in current code: > >> - if target gfn is used as its page table, guest will refault then kvm will use > >> small page size to map it. We need two #PF to fix its shadow page table > >> > >> - sometimes, say a exception is triggered during vm-exit caused by #PF > >> (see handle_exception() in vmx.c), we remove all the shadow pages shadowed > >> by the target gfn before go into page fault path, it will cause infinite > >> loop: > >> delete shadow pages shadowed by the gfn -> try to use large page size to map > >> the gfn -> retry the access ->... > >> > >> To fix these, We can adjust page size early if the target gfn is used as page > >> table > >> > >> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> > >> --- > >> arch/x86/kvm/mmu.c | 13 ++++--------- > >> arch/x86/kvm/paging_tmpl.h | 33 ++++++++++++++++++++++++++++++++- > >> 2 files changed, 36 insertions(+), 10 deletions(-) > >> > >> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c > >> index 2a3c890..54fc61e 100644 > >> --- a/arch/x86/kvm/mmu.c > >> +++ b/arch/x86/kvm/mmu.c > >> @@ -2380,15 +2380,10 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep, > >> if (pte_access & ACC_WRITE_MASK) { > >> > >> /* > >> - * There are two cases: > >> - * - the one is other vcpu creates new sp in the window > >> - * between mapping_level() and acquiring mmu-lock. > >> - * - the another case is the new sp is created by itself > >> - * (page-fault path) when guest uses the target gfn as > >> - * its page table. > >> - * Both of these cases can be fixed by allowing guest to > >> - * retry the access, it will refault, then we can establish > >> - * the mapping by using small page. > >> + * Other vcpu creates new sp in the window between > >> + * mapping_level() and acquiring mmu-lock. We can > >> + * allow guest to retry the access, the mapping can > >> + * be fixed if guest refault. > >> */ > >> if (level > PT_PAGE_TABLE_LEVEL && > >> has_wrprotected_page(vcpu->kvm, gfn, level)) > >> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h > >> index ec481e9..32d77ff 100644 > >> --- a/arch/x86/kvm/paging_tmpl.h > >> +++ b/arch/x86/kvm/paging_tmpl.h > >> @@ -491,6 +491,36 @@ out_gpte_changed: > >> return 0; > >> } > >> > >> + /* > >> + * To see whether the mapped gfn can write its page table in the current > >> + * mapping. > >> + * > >> + * It is the helper function of FNAME(page_fault). When guest uses large page > >> + * size to map the writable gfn which is used as current page table, we should > >> + * force kvm to use small page size to map it because new shadow page will be > >> + * created when kvm establishes shadow page table that stop kvm using large > >> + * page size. Do it early can avoid unnecessary #PF and emulation. > >> + * > >> + * 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(mapped_gfn_can_write_current_pagetable)(struct guest_walker *walker) > >> +{ > >> + int level; > >> + gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1); > >> + > >> + if (!(walker->pte_access & ACC_WRITE_MASK)) > >> + return false; > >> + > >> + for (level = walker->level; level <= walker->max_level; level++) > >> + if (!((walker->gfn ^ walker->table_gfn[level - 1]) & mask)) > >> + return true; > > > > XOR won't work. Just check with sums and integer comparison, ie. > > walker->gfn + KVM_PAGES_PER_HPAGE(walker->level). > > It can not work since walker->gfn is not large-page-size aligned. For example, > guest uses large page size to map 0x123000000 to physical address 0-2M, if > guest faults on 0x123001000, walker->gfn = 0x1000. > > The code "if (!((walker->gfn ^ walker->table_gfn[level - 1]) & mask))" is the > same as "if (walker->gfn & mask == walker->table_gfn[level - 1] & mask)" - if > any page in the large page area used as page table, we should use 4K page size > to fix it. > > In above example, if table_gfn is in the area [0, 2M), kvm is forced to use > 4k page size. Right, i misread it. > > Moreover, its confusing to have it checked at this level. What about > > doing at reexecute_instruction? > > Hmm, this patch is trying to fix a bug described in the changelog: > > ====== > - sometimes, say a exception is triggered during vm-exit caused by #PF > (see handle_exception() in vmx.c), we remove all the shadow pages shadowed > by the target gfn before go into page fault path, it will cause infinite > loop: > delete shadow pages shadowed by the gfn -> try to use large page size to map > the gfn -> retry the access ->... > ====== > > Which is caused by this code: > > if (is_page_fault(intr_info)) { > /* EPT won't cause page fault directly */ > BUG_ON(enable_ept); > cr2 = vmcs_readl(EXIT_QUALIFICATION); > trace_kvm_page_fault(cr2, error_code); > > if (kvm_event_needs_reinjection(vcpu)) > kvm_mmu_unprotect_page_virt(vcpu, cr2); > return kvm_mmu_page_fault(vcpu, cr2, error_code, NULL, 0); > } > > This bug is introduced in commit c219346325. > > Another way to fix it is doing this change: > @@ -2395,7 +2395,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep, > */ > if (level > PT_PAGE_TABLE_LEVEL && > has_wrprotected_page(vcpu->kvm, gfn, level)) > - goto done; > + return 1; > > The disadvantage of this way is, it causes unnecessary emulation. For example, > if 0-2M is mapped in guest and only page 0 used as page table, any write to > [4K, 2M) will need be emulated. > > Your idea? OK, i understand now. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html