| Message ID | 20150408121648.GA3519@potion.brq.redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 08/04/2015 14:16, Radim Kr?má? wrote: > 2015-04-08 12:43+0200, Paolo Bonzini: >> On 08/04/2015 11:26, Radim Kr?má? wrote: >>> Only cross-memslot writes have NULL memslot. >> >> The power of wrong comments... >> >> Considering how kvm_gfn_to_hva_cache_init is used (one 1-byte field, two >> 4-byte fields, one 28-bytes struct that is 32-bytes aligned, one >> 32-bytes field that is in practice cacheline-aligned), I wonder if we >> should just use ghc->memslot = NULL for cross page writes. This would >> bypass the bug you are fixing here, and avoid worries about partial writes. > > Good idea, and it could make those comments right :) > (Though in general, I prefer less constraints on APIs ...) It doesn't put constraints, it still handles cross page writes right (just slower). copy_to_user in some sense is the API that constrains us to do this. > Partial writes would be a pain; copy_to_user API does not define which > bytes were not written. I think the write can't fail mid-page, which > makes our implementation ok No, writes can't fail mid-page (I guess in atomic context it's theoretically possible, but we're equipped to handle the failure in that case). Patch applied, thanks! Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Apr 08, 2015 at 02:16:48PM +0200, Radim Kr?má? wrote: >2015-04-08 12:43+0200, Paolo Bonzini: >> On 08/04/2015 11:26, Radim Kr?má? wrote: >>> 2015-04-08 10:49+0200, Paolo Bonzini: >>>> On 07/04/2015 22:34, Radim Kr?má? wrote: >>>>> We dirtied only one page because writes originally couldn't span more. >>>>> Use improved syntax for '>> PAGE_SHIFT' while at it. >>>>> >>>>> Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.") >>>>> Signed-off-by: Radim Kr?má? <rkrcmar@redhat.com> >>>> >>>> Cross-page reads and writes should never get here; they have >>>> ghc->memslot set to NULL and go through the slow path in kvm_write_guest. >>> >>> Only cross-memslot writes have NULL memslot. >> >> The power of wrong comments... >> >> Considering how kvm_gfn_to_hva_cache_init is used (one 1-byte field, two >> 4-byte fields, one 28-bytes struct that is 32-bytes aligned, one >> 32-bytes field that is in practice cacheline-aligned), I wonder if we >> should just use ghc->memslot = NULL for cross page writes. This would >> bypass the bug you are fixing here, and avoid worries about partial writes. > >Good idea, and it could make those comments right :) >(Though in general, I prefer less constraints on APIs ...) > >Partial writes would be a pain; copy_to_user API does not define which >bytes were not written. I think the write can't fail mid-page, which >makes our implementation ok, but I still worry a bit about it. > >Anyway, here's the patch: > >---8<--- >kvm_write_guest_cached() does not mark all written pages as dirty and >code comments in kvm_gfn_to_hva_cache_init() talk about NULL memslot >with cross page accesses. Fix all the easy way. > >The check is '<= 1' to have the same result for 'len = 0' cache anywhere >in the page. (nr_pages_needed is 0 on page boundary.) > >Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.") >Signed-off-by: Radim Kr?má? <rkrcmar@redhat.com> Reviewed-by: Wanpeng Li <wanpeng.li@linux.intel.com> >--- > virt/kvm/kvm_main.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > >diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c >index aadef264bed1..f3dc641f9640 100644 >--- a/virt/kvm/kvm_main.c >+++ b/virt/kvm/kvm_main.c >@@ -1637,8 +1637,8 @@ int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc, > ghc->generation = slots->generation; > ghc->len = len; > ghc->memslot = gfn_to_memslot(kvm, start_gfn); >- ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn, &nr_pages_avail); >- if (!kvm_is_error_hva(ghc->hva) && nr_pages_avail >= nr_pages_needed) { >+ ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn, NULL); >+ if (!kvm_is_error_hva(ghc->hva) && nr_pages_needed <= 1) { > ghc->hva += offset; > } else { > /* >-- >To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >the body of a message to majordomo@vger.kernel.org >More majordomo info at http://vger.kernel.org/majordomo-info.html >Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index aadef264bed1..f3dc641f9640 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1637,8 +1637,8 @@ int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc, ghc->generation = slots->generation; ghc->len = len; ghc->memslot = gfn_to_memslot(kvm, start_gfn); - ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn, &nr_pages_avail); - if (!kvm_is_error_hva(ghc->hva) && nr_pages_avail >= nr_pages_needed) { + ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn, NULL); + if (!kvm_is_error_hva(ghc->hva) && nr_pages_needed <= 1) { ghc->hva += offset; } else { /*