Messages in this thread | | | From | Paul Durrant <> | Date | Wed, 22 Nov 2023 15:52:58 +0000 | Subject | Re: [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and use it consistently |
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On 22/11/2023 15:42, David Woodhouse wrote: > On Wed, 2023-11-22 at 22:27 +0800, Xu Yilun wrote: >> On Wed, Nov 22, 2023 at 09:12:18AM +0000, David Woodhouse wrote: >>> On Wed, 2023-11-22 at 16:54 +0800, Xu Yilun wrote: >>>> >>>>> @@ -259,13 +258,25 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, >>>>> ret = -EFAULT; >>>>> goto out; >>>>> } >>>>> + >>>>> + hva_change = true; >>>>> + } else { >>>>> + /* >>>>> + * No need to do any re-mapping if the only thing that has >>>>> + * changed is the page offset. Just page align it to allow the >>>>> + * new offset to be added in. >>>> >>>> I don't understand how the uhva('s offset) could be changed when both gpa and >>>> slot are not changed. Maybe I have no knowledge of xen, but in later >>>> patch you said your uhva would never change... >>> >>> It doesn't change on a normal refresh with kvm_gpc_refresh(), which is >>> just for revalidation after memslot changes or MMU invalidation. >>> >>> But it can change if the gpc is being reinitialized with a new address >>> (perhaps because the guest has made another hypercall to set the >>> address, etc.) >>> >>> That new address could happen to be in the *same* page as the previous >> >> In this case, the lower bits of new gpa should be different to gpc->gpa, >> so will hit "if (gpc->gpa != gpa ...)" branch. > > I think that 'if (gpc->gpa != gpa); branch is also gratuitously > refreshing when it doesn't need to; it only needs to refresh if the > *aligned* gpas don't match. >
I did look at that but decided that gfn_to_hva_memslot() was sufficiently lightweight that it was not really worth optimising.
> But it was like that already, so I won't heckle Paul any further :)
I appreciate it! :-)
Paul
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