Commit fbc7e611 authored by Mark Rutland's avatar Mark Rutland Committed by Marc Zyngier
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KVM: arm64: Unconditionally save+flush host FPSIMD/SVE/SME state 

There are several problems with the way hyp code lazily saves the host's
FPSIMD/SVE state, including:

* Host SVE being discarded unexpectedly due to inconsistent
  configuration of TIF_SVE and CPACR_ELx.ZEN. This has been seen to
  result in QEMU crashes where SVE is used by memmove(), as reported by
  Eric Auger:

  https://issues.redhat.com/browse/RHEL-68997



* Host SVE state is discarded *after* modification by ptrace, which was an
  unintentional ptrace ABI change introduced with lazy discarding of SVE state.

* The host FPMR value can be discarded when running a non-protected VM,
  where FPMR support is not exposed to a VM, and that VM uses
  FPSIMD/SVE. In these cases the hyp code does not save the host's FPMR
  before unbinding the host's FPSIMD/SVE/SME state, leaving a stale
  value in memory.

Avoid these by eagerly saving and "flushing" the host's FPSIMD/SVE/SME
state when loading a vCPU such that KVM does not need to save any of the
host's FPSIMD/SVE/SME state. For clarity, fpsimd_kvm_prepare() is
removed and the necessary call to fpsimd_save_and_flush_cpu_state() is
placed in kvm_arch_vcpu_load_fp(). As 'fpsimd_state' and 'fpmr_ptr'
should not be used, they are set to NULL; all uses of these will be
removed in subsequent patches.

Historical problems go back at least as far as v5.17, e.g. erroneous
assumptions about TIF_SVE being clear in commit:

  8383741a ("KVM: arm64: Get rid of host SVE tracking/saving")

... and so this eager save+flush probably needs to be backported to ALL
stable trees.

Fixes: 93ae6b01 ("KVM: arm64: Discard any SVE state when entering KVM guests")
Fixes: 8c845e27 ("arm64/sve: Leave SVE enabled on syscall if we don't context switch")
Fixes: ef3be860 ("KVM: arm64: Add save/restore support for FPMR")
Reported-by: default avatarEric Auger <eauger@redhat.com>
Reported-by: default avatarWilco Dijkstra <wilco.dijkstra@arm.com>
Reviewed-by: default avatarMark Brown <broonie@kernel.org>
Tested-by: default avatarMark Brown <broonie@kernel.org>
Tested-by: default avatarEric Auger <eric.auger@redhat.com>
Acked-by: default avatarWill Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Jeremy Linton <jeremy.linton@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
Reviewed-by: default avatarOliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250210195226.1215254-2-mark.rutland@arm.com


Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
parent 8dbccafc

arch/arm64/kernel/fpsimd.c

+0 −25
Original line number Diff line number Diff line
@@ -1694,31 +1694,6 @@ void fpsimd_signal_preserve_current_state(void) 
		sve_to_fpsimd(current);

}



/*

 * Called by KVM when entering the guest.

 */

void fpsimd_kvm_prepare(void)

{

	if (!system_supports_sve())

		return;



	/*

	 * KVM does not save host SVE state since we can only enter

	 * the guest from a syscall so the ABI means that only the

	 * non-saved SVE state needs to be saved.  If we have left

	 * SVE enabled for performance reasons then update the task

	 * state to be FPSIMD only.

	 */

	get_cpu_fpsimd_context();



	if (test_and_clear_thread_flag(TIF_SVE)) {

		sve_to_fpsimd(current);

		current->thread.fp_type = FP_STATE_FPSIMD;

	}



	put_cpu_fpsimd_context();

}



/*

 * Associate current's FPSIMD context with this cpu

 * The caller must have ownership of the cpu FPSIMD context before calling

arch/arm64/kvm/fpsimd.c

+10 −25
Original line number Diff line number Diff line
@@ -54,16 +54,18 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu) 
	if (!system_supports_fpsimd())

		return;



	fpsimd_kvm_prepare();



	/*

	 * We will check TIF_FOREIGN_FPSTATE just before entering the

	 * guest in kvm_arch_vcpu_ctxflush_fp() and override this to

	 * FP_STATE_FREE if the flag set.

	 * Ensure that any host FPSIMD/SVE/SME state is saved and unbound such

	 * that the host kernel is responsible for restoring this state upon

	 * return to userspace, and the hyp code doesn't need to save anything.

	 *

	 * When the host may use SME, fpsimd_save_and_flush_cpu_state() ensures

	 * that PSTATE.{SM,ZA} == {0,0}.

	 */

	*host_data_ptr(fp_owner) = FP_STATE_HOST_OWNED;

	*host_data_ptr(fpsimd_state) = kern_hyp_va(&current->thread.uw.fpsimd_state);

	*host_data_ptr(fpmr_ptr) = kern_hyp_va(&current->thread.uw.fpmr);

	fpsimd_save_and_flush_cpu_state();

	*host_data_ptr(fp_owner) = FP_STATE_FREE;

	*host_data_ptr(fpsimd_state) = NULL;

	*host_data_ptr(fpmr_ptr) = NULL;



	host_data_clear_flag(HOST_SVE_ENABLED);

	if (read_sysreg(cpacr_el1) & CPACR_EL1_ZEN_EL0EN)
@@ -73,23 +75,6 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu) 
		host_data_clear_flag(HOST_SME_ENABLED);

		if (read_sysreg(cpacr_el1) & CPACR_EL1_SMEN_EL0EN)

			host_data_set_flag(HOST_SME_ENABLED);



		/*

		 * If PSTATE.SM is enabled then save any pending FP

		 * state and disable PSTATE.SM. If we leave PSTATE.SM

		 * enabled and the guest does not enable SME via

		 * CPACR_EL1.SMEN then operations that should be valid

		 * may generate SME traps from EL1 to EL1 which we

		 * can't intercept and which would confuse the guest.

		 *

		 * Do the same for PSTATE.ZA in the case where there

		 * is state in the registers which has not already

		 * been saved, this is very unlikely to happen.

		 */

		if (read_sysreg_s(SYS_SVCR) & (SVCR_SM_MASK | SVCR_ZA_MASK)) {

			*host_data_ptr(fp_owner) = FP_STATE_FREE;

			fpsimd_save_and_flush_cpu_state();

		}

	}



	/*