From 4bedbe47eef4ff68265a63d89ab55401334424b2 Mon Sep 17 00:00:00 2001 From: Andrew Cooper Date: Mon, 23 Feb 2026 15:38:14 +0000 Subject: [PATCH] x86/emul: Remove fallback path from SWAPGS In real hardware, accesses to the registers cannot fail. The error paths are just an artefact of the hook functions needing to return something. The best effort unwind is also something that doesn't exist in real hardware, and makes the logic more complicated to follow. Instead, use an ASSERT_UNREACHABLE() with a fallback of injecting #DF. Hitting this path is an error in Xen. While adjusting, remove {read,write}_segment() and use {read,write}_msr() to access MSR_GS_BASE. There's no need to access the other parts of the GS segment, and this is less work behind the scenes. Signed-off-by: Andrew Cooper Reviewed-by: Jan Beulich --- xen/arch/x86/x86_emulate/0f01.c | 28 +++++++++++++++------------- 1 file changed, 15 insertions(+), 13 deletions(-) diff --git a/xen/arch/x86/x86_emulate/0f01.c b/xen/arch/x86/x86_emulate/0f01.c index 6c10979dd6..54bd6faf0f 100644 --- a/xen/arch/x86/x86_emulate/0f01.c +++ b/xen/arch/x86/x86_emulate/0f01.c @@ -189,22 +189,24 @@ int x86emul_0f01(struct x86_emulate_state *s, generate_exception_if(!mode_ring0(), X86_EXC_GP, 0); fail_if(!ops->read_segment || !ops->read_msr || !ops->write_segment || !ops->write_msr); - if ( (rc = ops->read_segment(x86_seg_gs, &sreg, - ctxt)) != X86EMUL_OKAY || - (rc = ops->read_msr(MSR_SHADOW_GS_BASE, &msr_val, + if ( (rc = ops->read_msr(MSR_GS_BASE, &sreg.base, ctxt)) != X86EMUL_OKAY || - (rc = ops->write_msr(MSR_SHADOW_GS_BASE, sreg.base, + (rc = ops->read_msr(MSR_SHADOW_GS_BASE, &msr_val, + ctxt)) != X86EMUL_OKAY ) + goto done; + if ( (rc = ops->write_msr(MSR_SHADOW_GS_BASE, sreg.base, + ctxt, false)) != X86EMUL_OKAY || + (rc = ops->write_msr(MSR_GS_BASE, msr_val, ctxt, false)) != X86EMUL_OKAY ) - goto done; - sreg.base = msr_val; - if ( (rc = ops->write_segment(x86_seg_gs, &sreg, - ctxt)) != X86EMUL_OKAY ) { - /* Best effort unwind (i.e. no real error checking). */ - if ( ops->write_msr(MSR_SHADOW_GS_BASE, msr_val, - ctxt, false) == X86EMUL_EXCEPTION ) - x86_emul_reset_event(ctxt); - goto done; + /* + * In real hardware, access to the registers cannot fail. It is + * an error in Xen if the writes fail given that both MSRs have + * equivalent checks. + */ + ASSERT_UNREACHABLE(); + x86_emul_reset_event(ctxt); + generate_exception(X86_EXC_DF, 0); } break;