From: Paul Durrant <xadimgnik@gmail.com>
To: David Woodhouse <dwmw2@infradead.org>,
Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and use it consistently
Date: Wed, 22 Nov 2023 09:29:46 +0000 [thread overview]
Message-ID: <04530097-a3b2-4acf-bf41-fba48143d4e1@xen.org> (raw)
In-Reply-To: <5ffdf3ab49b047cd851289e5dc0697af0ffff45f.camel@infradead.org>
On 21/11/2023 22:35, David Woodhouse wrote:
> On Tue, 2023-11-21 at 18:02 +0000, Paul Durrant wrote:
>> @@ -242,8 +242,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
>> }
>>
>> old_pfn = gpc->pfn;
>> - old_khva = gpc->khva - offset_in_page(gpc->khva);
>> - old_uhva = gpc->uhva;
>> + old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
>>
>> /* If the userspace HVA is invalid, refresh that first */
>> if (gpc->gpa != gpa || gpc->generation != slots->generation ||
>> @@ -259,13 +258,25 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
>> ret = -EFAULT;
>> goto out;
>> }
>
>
> There's a subtle behaviour change here, isn't there? I'd *really* like
> you do say 'No functional change intended' where that is true, and then
> the absence of that sentence in this one would be meaningful.
>
> You are now calling hva_to_pfn_retry() even when the uhva page hasn't
> changed. Which is harmless and probably not important, but IIUC fixable
> by the addition of:
>
> + if (gpc->uhva != PAGE_ALIGN_DOWN(old_uhva))
True; I can keep that optimization and then I will indeed add 'no
functional change'... Didn't seem worth it at the time, but no harm.
>> + 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.
>> + */
>> + gpc->uhva = PAGE_ALIGN_DOWN(gpc->uhva);
>> }
>>
>> + /* Note: the offset must be correct before calling hva_to_pfn_retry() */
>> + gpc->uhva += page_offset;
>> +
>> /*
>> * If the userspace HVA changed or the PFN was already invalid,
>> * drop the lock and do the HVA to PFN lookup again.
>> */
>> - if (!gpc->valid || old_uhva != gpc->uhva) {
>> + if (!gpc->valid || hva_change) {
>> ret = hva_to_pfn_retry(gpc);
>> } else {
>> /*
>> --
>
> But I don't really think it's that important if you can come up with a
> coherent justification for the change and note it in the commit
> message. So either way:
>
> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Thanks,
Paul
next prev parent reply other threads:[~2023-11-22 9:29 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-21 18:02 [PATCH v8 00/15] KVM: xen: update shared_info and vcpu_info handling Paul Durrant
2023-11-21 18:02 ` [PATCH v8 01/15] KVM: pfncache: Add a map helper function Paul Durrant
2023-11-21 18:02 ` [PATCH v8 02/15] KVM: pfncache: remove unnecessary exports Paul Durrant
2023-11-21 21:49 ` David Woodhouse
2023-11-22 8:44 ` Paul Durrant
2023-11-21 18:02 ` [PATCH v8 03/15] KVM: xen: mark guest pages dirty with the pfncache lock held Paul Durrant
2023-11-21 21:49 ` David Woodhouse
2023-11-21 18:02 ` [PATCH v8 04/15] KVM: pfncache: add a mark-dirty helper Paul Durrant
2023-11-21 18:02 ` [PATCH v8 05/15] KVM: pfncache: remove KVM_GUEST_USES_PFN usage Paul Durrant
2023-11-21 22:24 ` David Woodhouse
2023-11-27 23:36 ` Sean Christopherson
2023-11-21 18:02 ` [PATCH v8 06/15] KVM: pfncache: stop open-coding offset_in_page() Paul Durrant
2023-11-21 22:26 ` David Woodhouse
2023-11-21 18:02 ` [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and use it consistently Paul Durrant
2023-11-21 22:35 ` David Woodhouse
2023-11-22 9:29 ` Paul Durrant [this message]
2023-11-22 8:54 ` Xu Yilun
2023-11-22 9:12 ` David Woodhouse
2023-11-22 14:27 ` Xu Yilun
2023-11-22 15:42 ` David Woodhouse
2023-11-22 15:52 ` Paul Durrant
2023-11-21 18:02 ` [PATCH v8 08/15] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA Paul Durrant
2023-11-21 22:47 ` David Woodhouse
2023-11-22 10:07 ` Paul Durrant
2023-11-21 18:02 ` [PATCH v8 09/15] KVM: xen: allow shared_info to be mapped by fixed HVA Paul Durrant
2023-11-21 18:02 ` [PATCH v8 10/15] KVM: xen: allow vcpu_info " Paul Durrant
2023-11-21 18:02 ` [PATCH v8 11/15] KVM: selftests / xen: map shared_info using HVA rather than GFN Paul Durrant
2023-11-21 18:02 ` [PATCH v8 12/15] KVM: selftests / xen: re-map vcpu_info using HVA rather than GPA Paul Durrant
2023-11-21 18:02 ` [PATCH v8 13/15] KVM: xen: advertize the KVM_XEN_HVM_CONFIG_SHARED_INFO_HVA capability Paul Durrant
2023-11-21 18:02 ` [PATCH v8 14/15] KVM: xen: split up kvm_xen_set_evtchn_fast() Paul Durrant
2023-11-21 22:49 ` David Woodhouse
2023-11-21 18:02 ` [PATCH v8 15/15] KVM: xen: allow vcpu_info content to be 'safely' copied Paul Durrant
2023-11-21 22:53 ` David Woodhouse
2023-11-22 10:39 ` David Woodhouse
2023-11-22 10:55 ` Paul Durrant
2023-11-22 11:25 ` David Woodhouse
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