kvm_write_guest would work, but it will hurt performance a bit because it'll be doing the address translation each time the time is updated, which happens on most guest enters. Another possibility would be to change kvm_gfn_to_hva_cache_init to accept a size parameter. If the requested range is all on one page then it operates the same as it currently does. If the address range is on more than one page then it falls back to kvm_write_guest. This preserves the good performance for all cases that currently work, while still supporting the unlikely case of page straddling requests. It also makes it harder to write a security bugs for other callers of kvm_gfn_to_hva_cache_init by explicitly requiring a size parameter. I can write a patch if you like the idea. Andy On Fri, Mar 22, 2013 at 2:57 PM, Gleb Natapov <gleb@xxxxxxxxxx> wrote: > On Fri, Mar 22, 2013 at 06:19:47PM -0300, Marcelo Tosatti wrote: >> >> The alignment check is not necessary given that "KVM: x86: Convert >> MSR_KVM_SYSTEM_TIME to use gfn_to_hva_cache" (0b79459b482e85cb742) >> uses kvm_write_guest which is able to handle data across page >> properly. >> > It uses kvm_write_guest_cached which calls to copy_to_user() directly. > While this will not allow to overwrite page that does not belong to qemu > process, it is possible to write outside of memory slot. May be we > should use kvm_write_guest to do system time updates. > >> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index f19ac0a..ec830fa 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -1952,10 +1952,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info) >> >> gpa_offset = data & ~(PAGE_MASK | 1); >> >> - /* Check that the address is 32-byte aligned. */ >> - if (gpa_offset & (sizeof(struct pvclock_vcpu_time_info) - 1)) >> - break; >> - >> if (kvm_gfn_to_hva_cache_init(vcpu->kvm, >> &vcpu->arch.pv_time, data & ~1ULL)) >> vcpu->arch.pv_time_enabled = false; > > -- > Gleb. -- 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
