linux-kernel - Re: [PATCH 02/21] KVM: Allow page-sized MMU caches to be initialized with custom 64-bit values

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Date: Mon, 13 May 2024 13:56:19 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>
Cc: Binbin Wu <binbin.wu@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, michael.roth@....com, 
	thomas.lendacky@....com, isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH 02/21] KVM: Allow page-sized MMU caches to be initialized
 with custom 64-bit values

On Mon, May 13, 2024, Isaku Yamahata wrote:
> On Tue, Mar 26, 2024 at 11:56:35PM +0800, Binbin Wu <binbin.wu@...ux.intel.com> wrote:
> > > > diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> > > > index d93f6522b2c3..827ecc0b7e10 100644
> > > > --- a/include/linux/kvm_types.h
> > > > +++ b/include/linux/kvm_types.h
> > > > @@ -86,6 +86,7 @@ struct gfn_to_pfn_cache {
> > > >   struct kvm_mmu_memory_cache {
> > > >       gfp_t gfp_zero;
> > > >       gfp_t gfp_custom;
> > > > +    u64 init_value;
> > > >       struct kmem_cache *kmem_cache;
> > > >       int capacity;
> > > >       int nobjs;
> > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > > index 9c99c9373a3e..c9828feb7a1c 100644
> > > > --- a/virt/kvm/kvm_main.c
> > > > +++ b/virt/kvm/kvm_main.c
> > > > @@ -401,12 +401,17 @@ static void kvm_flush_shadow_all(struct kvm *kvm)
> > > >   static inline void *mmu_memory_cache_alloc_obj(struct
> > > > kvm_mmu_memory_cache *mc,
> > > >                              gfp_t gfp_flags)
> > > >   {
> > > > +    void *page;
> > > > +
> > > >       gfp_flags |= mc->gfp_zero;
> > > >         if (mc->kmem_cache)
> > > >           return kmem_cache_alloc(mc->kmem_cache, gfp_flags);
> > > > -    else
> > > > -        return (void *)__get_free_page(gfp_flags);
> > > > +
> > > > +    page = (void *)__get_free_page(gfp_flags);
> > > > +    if (page && mc->init_value)
> > > > +        memset64(page, mc->init_value, PAGE_SIZE /
> > > > sizeof(mc->init_value));
> > 
> > Do we need a static_assert() to make sure mc->init_value is 64bit?
> 
> That's overkill because EPT entry is defined as 64bit and KVM uses u64 for it
> uniformly.

I'm pretty sure Binbin is talking about passing init_value to memset64(), not
about whether or not that suffices for EPT.  So I wouldn't say it's overkill.

However, I don't think a static assert is warranted.  Functionally, tracking
init_value as a u32 or even a u8 would be a-ok as it's a copy-by-value parameter
that won't be sign-extended or truncated.  I.e. the real reqiurement comes from
TDX wanting to set a 64-bit value.

And trying to set bit 63 in a 32-bit field _will_ make the compiler unhappy:

arch/x86/kvm/mmu/mmu.c: In function ‘kvm_mmu_create’:
include/vdso/bits.h:8:33: error: conversion from ‘long long unsigned int’ to ‘u32’ {aka ‘unsigned int’} changes value from ‘9223372036854775808’ to ‘0’ [-Werror=overflow]
    8 | #define BIT_ULL(nr)             (ULL(1) << (nr))
      |                                 ^
arch/x86/kvm/mmu/spte.h:162:33: note: in expansion of macro ‘BIT_ULL’
  162 | #define SHADOW_NONPRESENT_VALUE BIT_ULL(63)
      |                                 ^~~~~~~
arch/x86/kvm/mmu/mmu.c:6225:17: note: in expansion of macro ‘SHADOW_NONPRESENT_VALUE’
 6225 |                 SHADOW_NONPRESENT_VALUE;
      |                 ^~~~~~~~~~~~~~~~~~~~~~~


I suppose one could argue that changing init_value to a u128 could result in
undetected truncation, but IMO that firmly crosses into ridiculous territory.