On 11/9/22 01:53, Sean Christopherson wrote:
On Tue, Nov 08, 2022, Paolo Bonzini wrote:
This is needed in order to keep the number of arguments to 3 or less, after adding hsave_pa and spec_ctrl_intercepted. 32-bit builds only support passing three arguments in registers, fortunately all other data is reachable from the vcpu_svm struct.
Is it actually a problem if parameters are passed on the stack? The assembly code mostly creates a stack frame, i.e. %ebp can be used to pull values off the stack.
It's not, but given how little love 32-bit KVM receives, I prefer to stick to the subset of the ABI that is "equivalent" to 64-bit.
no one cares about 32-bit and I highly doubt a few extra PUSH+POP instructions will be noticeable.
Same reasoning (no one cares about 32-bits), different conclusions...
What fields are actually used is (like with any other function) "potentially all, you'll have to read the source code and in fact you can just read asm-offsets.c instead". What I mean is, I cannot offhand see or remember what fields are touched by svm_prepare_switch_to_guest, why would __svm_vcpu_run be any different?
It's different because if it were a normal C function, it would simply take @vcpu, and maybe @spec_ctrl_intercepted to shave cycles after CLGI.
Not just for that, but especially to avoid making msr_write_intercepted() noinstr.
But because it's assembly and doesn't have to_svm() readily available (among other restrictions), __svm_vcpu_run() ends up taking a mishmash of parameters, which for me makes it rather difficult to understand what to expect.
Yeah, there could be three reasons to have parameters in assembly:
* you just need them (@svm)
* it's too much of a pain to compute it in assembly (@spec_ctrl_intercepted, @hsave_pa)
* it needs to be computed outside the clgi/stgi region (not happening here, only mentioned for completeness)
As this patch shows, @vmcb is not much of a pain to compute in assembly: it is just two instructions, and not passing it in simplifies register allocation (the weird push/pop goes away) because all the arguments except @svm/_ASM_ARG1 are needed only after vmexit.
Oooh, and after much staring I realized that the address of the host save area is passed in because grabbing it after VM-Exit can't work. That's subtle, and passing it in isn't strictly necessary; there's no reason the assembly code can't grab it and stash it on the stack.
Right, in fact that's not the reason why it's passed in---it's just to avoid coding page_to_pfn() in assembly, and to limit the differences between the regular and SEV-ES cases. But using a per-CPU variable is fine (either in addition to the struct page, which "wastes" 8 bytes per CPU, or as a replacement).
What about killing a few birds with one stone? Move the host save area PA to its own per-CPU variable, and then grab that from assembly as well.
I would still place it in struct svm_cpu_data itself, I'll see how it looks and possibly post v3.
Paolo