Date: Thu, 31 Aug 2006 08:18:57 -0700 (PDT) From: backyard <backyard1454-bsd@yahoo.com> To: "Chad Leigh -- Shire.Net LLC" <chad@shire.net>, freebsd-questions Questions <freebsd-questions@freebsd.org> Subject: Re: shared cache -- Re: SMP detection Message-ID: <20060831151857.42787.qmail@web83115.mail.mud.yahoo.com> In-Reply-To: <EF801204-411C-4C48-895A-1E6CD7BEA524@shire.net>
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--- "Chad Leigh -- Shire.Net LLC" <chad@shire.net> wrote: > > On Aug 30, 2006, at 12:12 PM, backyard wrote: > > > with HT disabling in FreeBSD is more for the > security > > issues about a potential exploit whereby one > process > > in one pipe can access the priveledged information > of > > a process in another pipe because the two cores > share > > one processor cache and thus one cache table. To > my > > knowledge this hasn't been exploited yet. > > > How is this any different than say an Intel Core Duo > or Core 2 Duo? > I believe they have a shared cache as well for each > (real) processor > core. > > Chad > > --- > Chad Leigh -- Shire.Net LLC > Your Web App and Email hosting provider > chad at shire.net > > I would say there is no difference if what you say is true. A Multi-Core chip is only true SMP if the two cores share no resources internally and thus are capable of running process separate from each other entirely. independantly and with their own internal caches. The process shouldn't have to wait on a lock to access it's cache, which I would have to assume occurs on these HT machines; which is probably why they have degraded performance. The cache should only be shared if a process explicity copies its content to the other cores cache. If should not be possible for both Cores to see the same internal cache. To my knowledge the AMDx2 follow this model with independant cores only sharing a common die. This ensures the context and priveledge of one running process cannot be compromised by a non-priveldeged process waiting on say a login attempt to root, and then grabbing the password from the common cache before the privelidged process can clean up. I don't think this flaw has been exploited yet, but the boys at OpenBSD found it (from memory, pretty sure it was one of them) and it has spread through the BSD community as it has potentially dire consequences. Personally I'm done with Intel so I don't think I'll ever have this issue. Afterall they're still the reason my computer boots up with 640k of RAM... I also think AMD has come from being a clone to being on top of the market, but this is my personal opinion. The fact Core Duos are only 32-bit means Intel is still only concerned with shortend gains on the Windows market, not long term migration to 64-bit PCs like everyone else... And banking on Microsoft has never been a solid idea; its too bad banks use Windows; there's a security nightmare, but a topic in and of itself... -brian
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