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-0700 In-Reply-To: Mime-Version: 1.0 References: <20250829-pmu_event_info-v5-0-9dca26139a33@rivosinc.com> <20250829-pmu_event_info-v5-6-9dca26139a33@rivosinc.com> Message-ID: Subject: Re: [PATCH v5 6/9] KVM: Add a helper function to check if a gpa is in writable memselot From: Sean Christopherson To: Atish Kumar Patra Cc: Anup Patel , Will Deacon , Mark Rutland , Paul Walmsley , Palmer Dabbelt , Mayuresh Chitale , linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250905_012327_680260_20900495 X-CRM114-Status: GOOD ( 25.31 ) X-BeenThere: kvm-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "kvm-riscv" 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<20250829-pmu_event_info-v5-0-9dca26139a33@rivosinc.com> <20250829-pmu_event_info-v5-6-9dca26139a33@rivosinc.com> Message-ID: Subject: Re: [PATCH v5 6/9] KVM: Add a helper function to check if a gpa is in writable memselot From: Sean Christopherson To: Atish Kumar Patra Cc: Anup Patel , Will Deacon , Mark Rutland , Paul Walmsley , Palmer Dabbelt , Mayuresh Chitale , linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Wed, Sep 03, 2025, Atish Kumar Patra wrote: > On Fri, Aug 29, 2025 at 1:47=E2=80=AFPM Sean Christopherson wrote: > > > > On Fri, Aug 29, 2025, Atish Patra wrote: > > > +static inline bool kvm_is_gpa_in_writable_memslot(struct kvm *kvm, g= pa_t gpa) > > > +{ > > > + bool writable; > > > + unsigned long hva =3D gfn_to_hva_prot(kvm, gpa_to_gfn(gpa), &wr= itable); > > > + > > > + return !kvm_is_error_hva(hva) && writable; > > > > I don't hate this API, but I don't love it either. Because knowing tha= t the > > _memslot_ is writable doesn't mean all that much. E.g. in this usage: > > > > hva =3D kvm_vcpu_gfn_to_hva_prot(vcpu, shmem >> PAGE_SHIFT, &wr= itable); > > if (kvm_is_error_hva(hva) || !writable) > > return SBI_ERR_INVALID_ADDRESS; > > > > ret =3D kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zer= o_sta)); > > if (ret) > > return SBI_ERR_FAILURE; > > > > the error code returned to the guest will be different if the memslot i= s read-only > > versus if the VMA is read-only (or not even mapped!). Unless every rea= d-only > > memslot is explicitly communicated as such to the guest, I don't see ho= w the guest > > can *know* that a memslot is read-only, so returning INVALID_ADDRESS in= that case > > but not when the underlying VMA isn't writable seems odd. > > > > It's also entirely possible the memslot could be replaced with a read-o= nly memslot > > after the check, or vice versa, i.e. become writable after being reject= ed. Is it > > *really* a problem to return FAILURE if the guest attempts to setup ste= al-time in > > a read-only memslot? I.e. why not do this and call it good? > > >=20 > Reposting the response as gmail converted my previous response as > html. Sorry for the spam. >=20 > From a functionality pov, that should be fine. However, we have > explicit error conditions for read only memory defined in the SBI STA > specification[1]. > Technically, we will violate the spec if we return FAILURE instead of > INVALID_ADDRESS for read only memslot. But KVM is already violating the spec, as kvm_vcpu_write_guest() redoes the memslot lookup and so could encounter a read-only memslot (if it races with a memslot update), and because the underlying memory could be read-only eve= n if the memslot is writable. Why not simply return SBI_ERR_INVALID_ADDRESS on kvm_vcpu_write_guest() fai= lure? The only downside of that is KVM will also return SBI_ERR_INVALID_ADDRESS i= f the userspace mapping is completely missing, but AFAICT that doesn't seem to be= an outright spec violation. 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Fri, 05 Sep 2025 01:23:26 -0700 (PDT) Date: Fri, 5 Sep 2025 01:23:17 -0700 In-Reply-To: Mime-Version: 1.0 References: <20250829-pmu_event_info-v5-0-9dca26139a33@rivosinc.com> <20250829-pmu_event_info-v5-6-9dca26139a33@rivosinc.com> Message-ID: Subject: Re: [PATCH v5 6/9] KVM: Add a helper function to check if a gpa is in writable memselot From: Sean Christopherson To: Atish Kumar Patra Cc: Anup Patel , Will Deacon , Mark Rutland , Paul Walmsley , Palmer Dabbelt , Mayuresh Chitale , linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250905_012327_428223_F0026DA8 X-CRM114-Status: GOOD ( 25.31 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org T24gV2VkLCBTZXAgMDMsIDIwMjUsIEF0aXNoIEt1bWFyIFBhdHJhIHdyb3RlOgo+IE9uIEZyaSwg QXVnIDI5LCAyMDI1IGF0IDE6NDfigK9QTSBTZWFuIENocmlzdG9waGVyc29uIDxzZWFuamNAZ29v Z2xlLmNvbT4gd3JvdGU6Cj4gPgo+ID4gT24gRnJpLCBBdWcgMjksIDIwMjUsIEF0aXNoIFBhdHJh IHdyb3RlOgo+ID4gPiArc3RhdGljIGlubGluZSBib29sIGt2bV9pc19ncGFfaW5fd3JpdGFibGVf bWVtc2xvdChzdHJ1Y3Qga3ZtICprdm0sIGdwYV90IGdwYSkKPiA+ID4gK3sKPiA+ID4gKyAgICAg Ym9vbCB3cml0YWJsZTsKPiA+ID4gKyAgICAgdW5zaWduZWQgbG9uZyBodmEgPSBnZm5fdG9faHZh X3Byb3Qoa3ZtLCBncGFfdG9fZ2ZuKGdwYSksICZ3cml0YWJsZSk7Cj4gPiA+ICsKPiA+ID4gKyAg ICAgcmV0dXJuICFrdm1faXNfZXJyb3JfaHZhKGh2YSkgJiYgd3JpdGFibGU7Cj4gPgo+ID4gSSBk b24ndCBoYXRlIHRoaXMgQVBJLCBidXQgSSBkb24ndCBsb3ZlIGl0IGVpdGhlci4gIEJlY2F1c2Ug a25vd2luZyB0aGF0IHRoZQo+ID4gX21lbXNsb3RfIGlzIHdyaXRhYmxlIGRvZXNuJ3QgbWVhbiBh bGwgdGhhdCBtdWNoLiAgRS5nLiBpbiB0aGlzIHVzYWdlOgo+ID4KPiA+ICAgICAgICAgaHZhID0g a3ZtX3ZjcHVfZ2ZuX3RvX2h2YV9wcm90KHZjcHUsIHNobWVtID4+IFBBR0VfU0hJRlQsICZ3cml0 YWJsZSk7Cj4gPiAgICAgICAgIGlmIChrdm1faXNfZXJyb3JfaHZhKGh2YSkgfHwgIXdyaXRhYmxl KQo+ID4gICAgICAgICAgICAgICAgIHJldHVybiBTQklfRVJSX0lOVkFMSURfQUREUkVTUzsKPiA+ Cj4gPiAgICAgICAgIHJldCA9IGt2bV92Y3B1X3dyaXRlX2d1ZXN0KHZjcHUsIHNobWVtLCAmemVy b19zdGEsIHNpemVvZih6ZXJvX3N0YSkpOwo+ID4gICAgICAgICBpZiAocmV0KQo+ID4gICAgICAg ICAgICAgICAgIHJldHVybiBTQklfRVJSX0ZBSUxVUkU7Cj4gPgo+ID4gdGhlIGVycm9yIGNvZGUg cmV0dXJuZWQgdG8gdGhlIGd1ZXN0IHdpbGwgYmUgZGlmZmVyZW50IGlmIHRoZSBtZW1zbG90IGlz IHJlYWQtb25seQo+ID4gdmVyc3VzIGlmIHRoZSBWTUEgaXMgcmVhZC1vbmx5IChvciBub3QgZXZl biBtYXBwZWQhKS4gIFVubGVzcyBldmVyeSByZWFkLW9ubHkKPiA+IG1lbXNsb3QgaXMgZXhwbGlj aXRseSBjb21tdW5pY2F0ZWQgYXMgc3VjaCB0byB0aGUgZ3Vlc3QsIEkgZG9uJ3Qgc2VlIGhvdyB0 aGUgZ3Vlc3QKPiA+IGNhbiAqa25vdyogdGhhdCBhIG1lbXNsb3QgaXMgcmVhZC1vbmx5LCBzbyBy ZXR1cm5pbmcgSU5WQUxJRF9BRERSRVNTIGluIHRoYXQgY2FzZQo+ID4gYnV0IG5vdCB3aGVuIHRo ZSB1bmRlcmx5aW5nIFZNQSBpc24ndCB3cml0YWJsZSBzZWVtcyBvZGQuCj4gPgo+ID4gSXQncyBh bHNvIGVudGlyZWx5IHBvc3NpYmxlIHRoZSBtZW1zbG90IGNvdWxkIGJlIHJlcGxhY2VkIHdpdGgg YSByZWFkLW9ubHkgbWVtc2xvdAo+ID4gYWZ0ZXIgdGhlIGNoZWNrLCBvciB2aWNlIHZlcnNhLCBp LmUuIGJlY29tZSB3cml0YWJsZSBhZnRlciBiZWluZyByZWplY3RlZC4gIElzIGl0Cj4gPiAqcmVh bGx5KiBhIHByb2JsZW0gdG8gcmV0dXJuIEZBSUxVUkUgaWYgdGhlIGd1ZXN0IGF0dGVtcHRzIHRv IHNldHVwIHN0ZWFsLXRpbWUgaW4KPiA+IGEgcmVhZC1vbmx5IG1lbXNsb3Q/ICBJLmUuIHdoeSBu b3QgZG8gdGhpcyBhbmQgY2FsbCBpdCBnb29kPwo+ID4KPiAKPiBSZXBvc3RpbmcgdGhlIHJlc3Bv bnNlIGFzIGdtYWlsIGNvbnZlcnRlZCBteSBwcmV2aW91cyByZXNwb25zZSBhcwo+IGh0bWwuIFNv cnJ5IGZvciB0aGUgc3BhbS4KPiAKPiBGcm9tIGEgZnVuY3Rpb25hbGl0eSBwb3YsIHRoYXQgc2hv dWxkIGJlIGZpbmUuIEhvd2V2ZXIsIHdlIGhhdmUKPiBleHBsaWNpdCBlcnJvciBjb25kaXRpb25z IGZvciByZWFkIG9ubHkgbWVtb3J5IGRlZmluZWQgaW4gdGhlIFNCSSBTVEEKPiBzcGVjaWZpY2F0 aW9uWzFdLgo+IFRlY2huaWNhbGx5LCB3ZSB3aWxsIHZpb2xhdGUgdGhlIHNwZWMgaWYgd2UgcmV0 dXJuIEZBSUxVUkUgaW5zdGVhZCBvZgo+IElOVkFMSURfQUREUkVTUyBmb3IgcmVhZCBvbmx5IG1l bXNsb3QuCgpCdXQgS1ZNIGlzIGFscmVhZHkgdmlvbGF0aW5nIHRoZSBzcGVjLCBhcyBrdm1fdmNw dV93cml0ZV9ndWVzdCgpIHJlZG9lcyB0aGUKbWVtc2xvdCBsb29rdXAgYW5kIHNvIGNvdWxkIGVu Y291bnRlciBhIHJlYWQtb25seSBtZW1zbG90IChpZiBpdCByYWNlcyB3aXRoCmEgbWVtc2xvdCB1 cGRhdGUpLCBhbmQgYmVjYXVzZSB0aGUgdW5kZXJseWluZyBtZW1vcnkgY291bGQgYmUgcmVhZC1v bmx5IGV2ZW4gaWYKdGhlIG1lbXNsb3QgaXMgd3JpdGFibGUuCgpXaHkgbm90IHNpbXBseSByZXR1 cm4gU0JJX0VSUl9JTlZBTElEX0FERFJFU1Mgb24ga3ZtX3ZjcHVfd3JpdGVfZ3Vlc3QoKSBmYWls dXJlPwpUaGUgb25seSBkb3duc2lkZSBvZiB0aGF0IGlzIEtWTSB3aWxsIGFsc28gcmV0dXJuIFNC SV9FUlJfSU5WQUxJRF9BRERSRVNTIGlmIHRoZQp1c2Vyc3BhY2UgbWFwcGluZyBpcyBjb21wbGV0 ZWx5IG1pc3NpbmcsIGJ1dCBBRkFJQ1QgdGhhdCBkb2Vzbid0IHNlZW0gdG8gYmUgYW4Kb3V0cmln aHQgc3BlYyB2aW9sYXRpb24uCgpfX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19f X19fX19fX19fXwpsaW51eC1yaXNjdiBtYWlsaW5nIGxpc3QKbGludXgtcmlzY3ZAbGlzdHMuaW5m cmFkZWFkLm9yZwpodHRwOi8vbGlzdHMuaW5mcmFkZWFkLm9yZy9tYWlsbWFuL2xpc3RpbmZvL2xp bnV4LXJpc2N2Cg==