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d="scan'208";a="15614112" Date: Tue, 14 Apr 2020 11:34:03 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Jan Beulich Subject: Re: [PATCH v9 1/3] x86/tlb: introduce a flush HVM ASIDs flag Message-ID: <20200414093403.GG28601@Air-de-Roger> References: <20200406105703.79201-1-roger.pau@citrix.com> <20200406105703.79201-2-roger.pau@citrix.com> <9c7ec98b-bd2d-4fbf-530a-2164dbbee200@suse.com> <20200408151055.GB28601@Air-de-Roger> <00c10f30-5502-2b43-b394-efa8137cf264@suse.com> <20200414080158.GD28601@Air-de-Roger> <106d7363-b341-f4a8-4771-589631c4690d@suse.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <106d7363-b341-f4a8-4771-589631c4690d@suse.com> X-ClientProxiedBy: AMSPEX02CAS02.citrite.net (10.69.22.113) To AMSPEX02CL02.citrite.net (10.69.22.126) X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: xen-devel@lists.xenproject.org, Tim Deegan , George Dunlap , Wei Liu , Andrew Cooper Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On Tue, Apr 14, 2020 at 11:09:43AM +0200, Jan Beulich wrote: > On 14.04.2020 10:01, Roger Pau Monné wrote: > > On Thu, Apr 09, 2020 at 01:16:57PM +0200, Jan Beulich wrote: > >> On 08.04.2020 17:10, Roger Pau Monné wrote: > >>> On Wed, Apr 08, 2020 at 01:25:14PM +0200, Jan Beulich wrote: > >>>> On 06.04.2020 12:57, Roger Pau Monne wrote: > >>>>> --- a/xen/arch/x86/mm/paging.c > >>>>> +++ b/xen/arch/x86/mm/paging.c > >>>>> @@ -613,7 +613,8 @@ void paging_log_dirty_range(struct domain *d, > >>>>> > >>>>> p2m_unlock(p2m); > >>>>> > >>>>> - flush_tlb_mask(d->dirty_cpumask); > >>>>> + flush_mask(d->dirty_cpumask, (!hap_enabled(d) ? FLUSH_TLB : 0) | > >>>>> + FLUSH_HVM_ASID_CORE); > >>>> > >>>> In cases where one case is assumed to be more likely than the other > >>>> putting the more likely one first can be viewed as a mild hint to > >>>> the compiler, and hence an extra ! may be warranted in an if() or > >>>> a conditional expression. Here, however, I don't think we can > >>>> really consider one case more likely than the other, and hence I'd > >>>> suggest to avoid the !, flipping the other two expressions > >>>> accordingly. I may take the liberty to adjust this while committing > >>>> (if I'm to be the one). > >>> > >>> That's fine, thanks. Somehow '!hap -> flush' was clearer in my mind. > >> > >> Thinking about it with the other HVM-related changes in v9, shouldn't > >> this then be > >> > >> flush_mask(d->dirty_cpumask, (hap_enabled(d) ? 0 : FLUSH_TLB) | > >> (is_hvm_domain(d) ? FLUSH_HVM_ASID_CORE : 0)); > >> > >> Or wait - the only caller lives in hap.c. As a result the FLUSH_TLB > >> part can be dropped altogether. And I question the need of flushing > >> guest TLBs - this is purely a p2m operation. I'll go look at the > >> history of this function, but for now I think the call should be > >> dropped (albeit then maybe better in a separate patch). > > > > The ASID flush needs to stay unless it's moved into p2m_pt_set_entry, > > as p2m_pt_set_entry itself doesn't perform any ASID flush and won't > > work correctly. > > Just like for said in the other reply sent a few minutes ago - yes > for NPT, but no for EPT. It's not strictly wrong for EPT as it won't cause EPT domains to malfunction, it's just redundant. > > I think it's safe to remove the TLB flush, as the code is only called > > from HAP, and hence is not used by shadow (which is what would require > > a plain TLB flush). The placement of this function seems misleading to > > me, as it looks like it's used by both shadow and HAP. It might be > > better to move it to hap.c if it's only to be used by HAP code. > > Either placement has its problems, I think. The function is meant to > be a paging layer one, but is needed by HAP only right now. I'm > pondering whether to wrap it in #ifdef CONFIG_HVM (plus perhaps a > respective ASSERT_UNREACHABLE()). IMO if a TLB flush is not performed here we should add an ASSERT_UNREACHABLE if called from a shadow mode domain, or else we risk someone trying to use it in shadow later without realizing it's missing a TLB flush. Thanks, Roger.