On May 1, 2025 11:30:18 AM PDT, Sean Christopherson <[email protected]> wrote: >On Thu, May 01, 2025, Peter Zijlstra wrote: >> On Thu, May 01, 2025 at 12:30:38PM +0200, Peter Zijlstra wrote: >> > On Wed, Apr 30, 2025 at 09:06:00PM +0200, Peter Zijlstra wrote: >> > > On Wed, Apr 30, 2025 at 07:24:15AM -0700, H. Peter Anvin wrote: >> > > >> > > > >KVM has another; the VMX interrupt injection stuff calls the IDT >> > > > >handler >> > > > >directly. Is there an alternative? Can we keep a table of Linux >> > > > >functions >> > > > >slighly higher up the call stack (asm_\cfunc ?) and add CFI to those? >> > > >> > > > We do have a table of handlers higher up in the stack in the form of >> > > > the dispatch tables for FRED. They don't in general even need the >> > > > assembly entry stubs, either. >> > > >> > > Oh, right. I'll go have a look at those. >> > >> > Right, so perhaps the easiest way around this is to setup the FRED entry >> > tables unconditionally, have VMX mandate CONFIG_FRED and then have it >> > always use the FRED entry points. >> > >> > Let me see how ugly that gets. >> >> Something like so... except this is broken. Its reporting spurious >> interrupts on vector 0x00, so something is buggered passing that vector >> along. > >Uh, aren't you making this way more complex than it needs to be? IIUC, KVM >never >uses the FRED hardware entry points, i.e. the FRED entry tables don't need to >be >in place because they'll never be used. The only bits of code KVM needs is the >__fred_entry_from_kvm() glue. > >Lightly tested, but this combo works for IRQs and NMIs on non-FRED hardware. > >-- >From 664468143109ab7c525c0babeba62195fa4c657e Mon Sep 17 00:00:00 2001 >From: Sean Christopherson <[email protected]> >Date: Thu, 1 May 2025 11:20:29 -0700 >Subject: [PATCH 1/2] x86/fred: Play nice with invoking > asm_fred_entry_from_kvm() on non-FRED hardware > >Modify asm_fred_entry_from_kvm() to allow it to be invoked by KVM even >when FRED isn't fully enabled, e.g. when running with CONFIG_X86_FRED=y >on non-FRED hardware. This will allow forcing KVM to always use the FRED >entry points for 64-bit kernels, which in turn will eliminate a rather >gross non-CFI indirect call that KVM uses to trampoline IRQs by doing IDT >lookups. > >When FRED isn't enabled, simply skip ERETS and restore RBP and RSP from >the stack frame prior to doing a "regular" RET back to KVM (in quotes >because of all the RET mitigation horrors). > >Signed-off-by: Sean Christopherson <[email protected]> >--- > arch/x86/entry/entry_64_fred.S | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > >diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S >index 29c5c32c16c3..7aff2f0a285f 100644 >--- a/arch/x86/entry/entry_64_fred.S >+++ b/arch/x86/entry/entry_64_fred.S >@@ -116,7 +116,8 @@ SYM_FUNC_START(asm_fred_entry_from_kvm) > movq %rsp, %rdi /* %rdi -> pt_regs */ > call __fred_entry_from_kvm /* Call the C entry point */ > POP_REGS >- ERETS >+ >+ ALTERNATIVE "", __stringify(ERETS), X86_FEATURE_FRED > 1: > /* > * Objtool doesn't understand what ERETS does, this hint tells it that >@@ -124,7 +125,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm) > * isn't strictly needed, but it's the simplest form. > */ > UNWIND_HINT_RESTORE >- pop %rbp >+ leave > RET > > SYM_FUNC_END(asm_fred_entry_from_kvm) > >base-commit: 45eb29140e68ffe8e93a5471006858a018480a45
Ok maybe I'm being dense, but what is left other than simply calling __fred_entry_from_kvm() as a normal C function? I'm on the go so there might be something in the code I'm missing, but on the surface...?
