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Peter Anvin" , Juergen Gross , Jiri Slaby , Kees Cook , kvm list , LKML , Thomas Hellstrom , Linux Virtualization , X86 ML , Sean Christopherson , Andrew Cooper Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace) Message-ID: <20200623104559.GA4817@hirez.programming.kicks-ass.net> References: <20200425191032.GK21900@8bytes.org> <910AE5B4-4522-4133-99F7-64850181FBF9@amacapital.net> <20200425202316.GL21900@8bytes.org> <20200623094519.GF31822@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200623094519.GF31822@suse.de> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 23, 2020 at 11:45:19AM +0200, Joerg Roedel wrote: > Hi Andy, > > On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote: > > 1. Use IST for #VC and deal with all the mess that entails. > > With the removal of IST shifting I wonder what you would suggest on how > to best implement an NMI-safe IST handler with nesting support. > > My current plan is to implement an IST handler which switches itself off > the IST stack as soon as possible, freeing it for re-use. > > The flow would be roughly like this upon entering the handler; > > build_pt_regs(); > > RSP = pt_regs->sp; > > if (RSP in VC_IST_stack) > error("unallowed nesting") > > if (RSP in current_kernel_stack) > RSP = round_down_to_8(RSP) > else > RSP = current_top_of_stack() // non-ist kernel stack > > copy_pt_regs(pt_regs, RSP); > switch_stack_to(RSP); > > To make this NMI safe, the NMI handler needs some logic too. Upon > entering NMI, it needs to check the return RSP, and if it is in the #VC > IST stack, it must do the above flow by itself and update the return RSP > and RIP. It needs to take into account the case when PT_REGS is not > fully populated on the return side. > > Alternativly the NMI handler could safe/restore the contents of the #VC > IST stack or just switch to a special #VC-in-NMI IST stack. > > All in all it could get complicated, and imho shift_ist would have been > simpler, but who am I anyway... > > Or maybe you have a better idea how to implement this, so I'd like to > hear your opinion first before I spend too many days implementing > something. OK, excuse my ignorance, but I'm not seeing how that IST shifting nonsense would've helped in the first place. If I understand correctly the problem is: <#VC> shift IST ... does stuff <#VC> # again, safe because the shift But what happens if you get the NMI before your IST adjustment? <#VC> ... does stuff <#VC> # again, happily wrecks your earlier #VC shift IST # whoopsy, too late Either way around we get to fix this up in NMI (and any other IST exception that can happen while in #VC, hello #MC). And more complexity there is the very last thing we need :-( There's no way you can fix up the IDT without getting an NMI first. This entire exception model is fundamentally buggered :-/