Re: [PATCH RFC V3 4/9] x86/pks: Preserve the PKRS MSR on context switch

On Tue, Oct 13, 2020 at 11:31:45AM -0700, Dave Hansen wrote:

...

+/**

• • It should also be noted that the underlying WRMSR(MSR_IA32_PKRS) is not
• • serializing but still maintains ordering properties similar to WRPKRU.
• • The current SDM section on PKRS needs updating but should be the same as
• • that of WRPKRU. So to quote from the WRPKRU text:
• • WRPKRU will never execute transiently. Memory accesses
• • affected by PKRU register will not execute (even transiently)
• • until all prior executions of WRPKRU have completed execution
• • and updated the PKRU register.
• */

+void write_pkrs(u32 new_pkrs) +{

• u32 *pkrs;
• if (!static_cpu_has(X86_FEATURE_PKS))

• return;
  

• pkrs = get_cpu_ptr(&pkrs_cache);
• if (*pkrs != new_pkrs) {

• *pkrs = new_pkrs;
  

• wrmsrl(MSR_IA32_PKRS, new_pkrs);
  

• }
• put_cpu_ptr(pkrs);

+}

It bugs me a *bit* that this is being called in a preempt-disabled region, but we still bother with the get/put_cpu jazz. Are there other future call-sites for this that aren't in preempt-disabled regions?

So the previous version had a useful comment that got lost. This stuff needs to fundamentally be preempt disabled, so it either needs to explicitly do so, or have an assertion that preemption is indeed disabled.